This should work ... just make sure you change the id on line 18 to go along with the products you want to be setup as your lifetime + trials

mepr_lifetime_trial.php

<?php
/*
Plugin Name: MemberPress Lifetime Trial
Plugin URI: http://memberpress.com
Description: Creates a lifetime subscription from a 1 year subscription + a *free* trial. This basically cancels the subscription after the first real payment
Version: 1.0.0
Author: Caseproof, LLC
Author URI: http://caseproof.com
Text Domain: memberpress
Copyright: 2004-2013, Caseproof, LLC
*/

add_action('mepr-txn-status-complete', 'mepr_lifetime_trial');
function mepr_lifetime_trial($txn) {
  // do this on the first real payment for
  // product_id==whatever that aren't free or
  // manual transactions and are attached to a subscription
  if( $txn->product_id==456 and
      $txn->txn_type==MeprTransaction::$payment_str and
      $txn->status==MeprTransaction::$complete_str and
      !in_array( $txn->gateway,
                 array( MeprTransaction::$free_gateway_str,
                        MeprTransaction::$manual_gateway_str ) ) )
  {
    // The first transaction for paid trials counts as a payment so in 
    // this scenario we want to only do this on the second payment method
    if( $sub = $txn->subscription() and 
        $sub->trial_amount > 0.00 and
        $sub->txn_count <= 1 )
    {
      return false;
    }

    // Update the transaction to have a never ending expiration date
    MeprTransaction::update( $txn->id, $txn->amount, $txn->user_id,
                             $txn->product_id, $txn->txn_type, $txn->status, $txn->coupon_id,
                             $txn->response, $txn->trans_num, $txn->subscription_id,
                             $txn->gateway, $txn->created_at, '0000-00-00 00:00:00' );

    // Cancel the subscription
    $pm = $txn->payment_method();
    $pm->process_cancel_subscription($sub->ID);

    return true;
  }
}

If you want to use multiple products change line 18 to something like
if(in_array($txn->product_id, array(456, 123, 309)) and

@cartpauj ... I finally got this updated. I haven't tested this code but it should look something like this.

I am getting an error when attempting to activate the plugin:

Parse error: syntax error, unexpected '}' in /wp-content/plugins/mepr_lifetime_trial.php on line 32

Line 31 is missing a semi colon

return false;

Added the semi-colon to the gist ...

I also think that the action needs to be changed to this

add_action('mepr-txn-status-complete', 'mepr_lifetime_trial', 10, 1);