When the user gets redirected from the protected content to the Membership Options page, some vars are available, which include the page he was redirected from.
WP Admin -> s2Member -> API / Scripting -> Membership Options Page VariablesYou could have a PHP script in your Membership Options page that stores that page in a cookie and then, after the person creates his account and logs in, have another PHP script in your login welcome page that checks if the cookie with the pre-registration page exists, and if so redirect him there.
If you don't know how to code all this, you could post a job in a freelance website like jobs.wordpress.net, oDesk.com or eLance.com.
I hope it helps.
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