Even if there aren't individual serial numbers (and I certainly don't think there are), it would be a simple matter to test it, make sure the game is working, and then give the code. Or at least it would be a simple matter if a) game stores wanted to participate in such a boondoggle that does little or nothing for their regular business (they wouldn't) and b) licensing agreements were in place with every publisher to make their titles available online (they aren't) and allow exchanges of physical discs for downloadable copies (they surely aren't).
Sony doesn't own every game released for the PSP. They were involved in the licensing and production processes, but in the absence of a contract to the contrary, they don't get to unilaterally determine alternate methods of distributing those games.
This is all a tempest in a teapot anyway. The PSP has never been significantly popular in the US and the issue of backwards compatibility will recede into the background as new games are released for the Vita (assuming it does well and generates publisher interest). In the meantime, getting a used PSP - or a new one, for that matter - is easy and cheap. Nobody with these mythical huge collections of UMD games is losing the ability to play their games, either now or in the foreseeable future. I don't understand why they'd have any interest in the Vita anyway if they're still playing the old games so much...oh wait, I do understand: It's fun to bitch about big companies in general, and Sony in particular.