gistfile1.php

<?php
/**
 * Add user meta to signify the user has just logged in
 * 
 */
add_action( 'wp_login', 'ac_fresh_login', 10, 2 );

function ac_fresh_login( $user_login, $user ) {

	update_user_meta( $user->ID, 'fresh_login', 1 );

}

/**
 * One-time Discourse SSO iframe
 * 
 */
add_action( 'wp_footer', 'ac_discourse_sso_iframe' );

function ac_discourse_sso_iframe() {

	// Only show iframe on first page the user hits after logging in
	
	$fresh_login = get_user_meta( get_current_user_id(), 'fresh_login', true );

	if ( $fresh_login == 1 ) {

		echo '<iframe src="http://discourse.example.com/session/sso" width="0" height="0" tabindex="-1" title="Discourse SSO" style="display:none" hidden>' . "\n";

		delete_user_meta( get_current_user_id(), 'fresh_login' );

	}

}