s2Member Pro-Form customizations that hide the username field and then auto-generate a username based on the email address. This was used for the Paid Journal subscription I ran on raamdev.com between 2011 and 2014.

raamdev-journal-s2-customizations.php

<?php
add_action ("login_head", "s2_customize_login", 1000);
function s2_customize_login ()
    {
?>
        <script type = "text/javascript">
		function getParameterByName(name)
		{
		  name = name.replace(/[\[]/, "\\\[").replace(/[\]]/, "\\\]");
		  var regexS = "[\\?&]" + name + "=([^&#]*)";
		  var regex = new RegExp(regexS);
		  var results = regex.exec(window.location.href);
		  if(results == null)
		    return "";
		  else
		    return decodeURIComponent(results[1].replace(/\+/g, " "));
		}

            (function ($) /* Wraps this `$ = jQuery` routine. */
                {
                    $.fn.swapWith = function (to) /* Utility extension for jQuery. */
                        {
                            return this.each (function ()
                                {
                                    var $to = $ (to).clone (true), $from = $ (this).clone (true);
                                    $(to).replaceWith ($from), $ (this).replaceWith ($to);
                                });
                        };
                    /**/
                    $(document).ready (function () /* Handles email-to-username on keyup. */
                        {	
							/* If this is the register page, customize it */
							if(getParameterByName('action') == 'register'){
	
								/* Generate random number to append to username, 
	                            hopefully making it unique (yes, this isn't perfect!) */
	                            var randomNum = Math.ceil(Math.random()*999);

	                            var email = 'input#user_email';
	                            var login = 'input#user_login';
	                            var firstname = 'input#ws-plugin--s2member-custom-reg-field-first-name';
	                            var lastname = 'input#ws-plugin--s2member-custom-reg-field-last-name';
	
	                            $('p.message').text("Thank you for your payment! Please complete your subscription below:");
	                            $('div.ws-plugin--s2member-custom-reg-field-divider-section').hide();
	                            $('#reg_passmail').hide();
	                            $('#nav').hide();
	                            $('#backtoblog').hide();
	                            $('#wp-submit').val('Submit');
								$(login).closest ('p').hide();
								
								$(email).closest ('p').swapWith ($ (firstname).closest ('p'));
								$(email).closest ('p').swapWith ($ (lastname).closest ('p'));
								$(firstname).focus();
								$(email).attr('tabindex', 50);
	                            /* Fill hidden username field with first part of email address
	                                and append randomNum to hopefully make it unique. */
	                            $ (email).keyup (function ()
	                                {
	                                    $(login).val ($.trim ($ (email).val ().split (/@/)[0].replace (/[^\w]/gi, '')) + randomNum.toString());
	                                });								
							}
                        });
                }) (jQuery);
        </script>
<?php
    }

add_action ("ws_plugin__s2member_pro_before_sc_paypal_form", "s2_customize_pro_form", 1000);
function s2_customize_pro_form ($vars = array())
    {
	/*  Don't pre populate when/if the Pro Form has just been POSTd, and/or if it's a Modification Form 
		with modify="1", because s2Member already pre populates Pro Forms in that scenario, with the 
		User's existing Email and Username. */
	if(empty ($_POST["s2member_pro_paypal_checkout"]) && empty($vars["attr"]["modify"])) {
?>
        <script type = "text/javascript">
		function getParameterByName(name)
		{
		  name = name.replace(/[\[]/, "\\\[").replace(/[\]]/, "\\\]");
		  var regexS = "[\\?&]" + name + "=([^&#]*)";
		  var regex = new RegExp(regexS);
		  var results = regex.exec(window.location.href);
		  if(results == null)
		    return "";
		  else
		    return decodeURIComponent(results[1].replace(/\+/g, " "));
		}
		
            (function ($) /* Wraps this `$ = jQuery` routine. */
                {
                    $.fn.swapWith = function (to) /* Utility extension for jQuery. */
                        {
                            return this.each (function ()
                                {
                                    var $to = $ (to).clone (true), $from = $ (this).clone (true);
                                    $(to).replaceWith ($from), $ (this).replaceWith ($to);
                                });
                        };
                    /**/
                    $(document).ready (function () /* Handles email-to-username on keyup. */
                        {	
								
						/* Generate random number to append to username, 
                        hopefully making it unique (yes, this isn't perfect!) */
                        var randomNum = Math.ceil(Math.random()*999);

                        if($('input#s2member-pro-paypal-checkout-email').length) {
							var email = 'input#s2member-pro-paypal-checkout-email';
							var login = 'input#s2member-pro-paypal-checkout-username';
							$('#s2member-pro-paypal-checkout-form-username-div').hide();
	                        $('#s2member-pro-paypal-checkout-form-description-section').hide();
							//$('div.s2member-pro-paypal-checkout-form-registration-section-title').text("Subscribe to Raam's Journal ($7/month)");
							if ( $('#pro-form-title').length ){ pro_form_title = $('#pro-form-title').html(); }
							//else { pro_form_title = "Raam's Journal Subscription"; }
							
							$('#s2member-pro-paypal-checkout-form-registration-section-title').text(pro_form_title);
						} else {
							var email = 'input#s2member-pro-paypal-registration-email';
							var login = 'input#s2member-pro-paypal-registration-username';
							$('#s2member-pro-paypal-registration-form-username-div').hide();
	                        $('#s2member-pro-paypal-registration-form-description-section').hide();
							$('div.s2member-pro-paypal-registration-form-registration-section-title').text("Complimentary Journal Subscription");
						}
						
						

                        /* Fill hidden username field with first part of email address
                            and append randomNum to hopefully make it unique. */
                        $ (email).keyup (function ()
                            {
                                $(login).val ($.trim ($ (email).val ().split (/@/)[0].replace (/[^\w]/gi, '')) + randomNum.toString());
                            });								
                        });
                }) (jQuery);
        </script>
<?php
		}
    }

add_action ("ws_plugin__s2member_pro_before_sc_paypal_form", "s2_customize_modify_pro_form", 1000);
function s2_customize_modify_pro_form ($vars = array())
    {
	/*  Only apply changes if it's a Modification Form with modify="1" */
	if(!empty($vars["attr"]["modify"])) {
?>
        <script type = "text/javascript">
            (function ($) /* Wraps this `$ = jQuery` routine. */
                {
                    $(document).ready (function () /* Handles email-to-username on keyup. */
                        {
							$('#s2member-pro-paypal-checkout-form-username-div').hide();
	                        $('#s2member-pro-paypal-checkout-form-description-section').hide();
							if ( $('#pro-form-title').length ){ pro_form_title = $('#pro-form-title').html(); }
							$('#s2member-pro-paypal-checkout-form-registration-section-title').text(pro_form_title);
                        });
                }) (jQuery);
        </script>
<?php
		}
    }

add_action ("ws_plugin__s2member_pro_before_sc_paypal_form", "s2_customize_sp_pro_form", 1000);
function s2_customize_sp_pro_form ($vars = array())
    { 
	?>
     <script type = "text/javascript">
         (function ($) /* Wraps this `$ = jQuery` routine. */
             {
                 $(document).ready (function () /* Handles email-to-username on keyup. */
                     {
                      $('#s2member-pro-paypal-sp-checkout-form-description-div').hide();
				if ( $('#pro-form-title').length ){ pro_form_title = $('#pro-form-title').html(); }
				$('#s2member-pro-paypal-sp-checkout-form-registration-section-title').text(pro_form_title);
                     });
             }) (jQuery);
     </script>
<?php
    }
?>