Brilliant. Thank you. I wish I had some big long reply, but just thank you. I'm going to chew on it for a while. If you have any other brilliant ideas, I am all ears. :)
Well, the bitch of the matter here is making it in a way that scales up so you don't either burn out to a crisp trying to keep up with the requirement of all those people wanting those background tidbits to mean something or making them be some inert piece of trivia in their +bg somewhere that barely does anything for them. After all if it doesn't generate RP what good is it?
You probably want to crowdsource this shit. Like, let people pick their poison from a list ("this is what's available for you guys"). Some of them will pick individual tidbits ("sure, my PC spent two years in that ultra conservative catholic school") waiting to be paired up with others pretty much at random and others might create as a unit ("we got really drunk that one night and broke into Native American museum"). Then try to organize - but not run yourself - that unicorn of all PrP arrangements; plot trading.
Plot trading is ... hah. It's not easy. I very often see people offering but it very rarely happens. So you'd need to be on top of people, motivating them any way you can at all - XP, recognition, extra access to staff plot, anything - to run stories for each other. Basically the idea (in this context, pending further refinement) would be to give players control over another unit's metaplot; that'd include a write-up of just what it entails (what did those drunken idiots steal? what did it do to them?), some loosely defined NPCs they can use (the now-retired teacher with a worm inside their skull, the former school janitor now working for an asylum) and let them go to town. Hopefully.
Basically you can't do too much to encourage plot on this level. Player-ran plot is the only thing here that scales up, where almost everything else does not.
Running plot is a lot of work. It just takes so long and there are so many bored players around. If they take care of each other that's great, but if you need to do it for them... well, you might as well book a spot for that asylum, too. :)