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
Make static backups of these sites using their sitemaps[0] and then ask them to donate or point their domains
[0] https://posterous.com/sitemap.xml (warning: 1.5MB)