Ideally, a "commitment" should be considered fulfilled the moment they meet that criteria, and they should retain that status regardless of how the site/private beta ultimately does. Their commitment has been fulfilled.
For users who haven't (yet) fulfilled their comment when a private beta shuts down, that data should just be discarded. We simply do not know if they would have fulfilled that commitment or not. It shouldn't be counted against them.
I may just be restating your feature-request, but I wanted to clarify what happens to the average user who likely did not have time to fulfill their commitment when a site closes down.