Catacaustic,
I do not think S2Member is an option (though I asked there because I own it), because it basically would mean that the people are signed up after they entered the code. Same, i guess, with Membership 2 Pro. I pretty much only would like a check if the code is valid and then have a cookie set to that the splash page is gone for now and for future visits.
My people should be needed to sing up the normal way, because this needs to be part of the testing process and because the signup seperates different user groups etc.
Marian