I've been keeping reocities alive out of pocket. It costs a few hundred every month in hosting costs and another few hundred in machine write-offs. I got about .37 in bitcoin donations, so I'm a bit short still :) But I think it is well worth doing. I assume hosing posterous would cost a lot more than that so I'd have to find some funding but I'm pretty sure this could be done.
I'd be more than happy to help out if Twitter are willing to do it with Posterous. Register a non-profit and then go out and approach some of the larger co's for funds and/or resources to keep it running forever.
Make static backups of these sites using their sitemaps[0] and then ask them to donate or point their domains
