Right now all purchases are associated with an email address, you can request a list of your purchases to be emailed to you from this page: http://itch.io/about/support

I'm very close to deploying a feature for registered users to view their purchases. I'm doing a revamp of user profiles and that's part of it.

Early access games are interesting, I think it's a realistic alternative to kickstarter. I don't have any rules regarding them, still thinking about it, but I very much encourage them.