Create 42000 chunks of 500 GB of the IA, each a zip file.
Make 42000 torrents.
Make an interface to suggest a torrent, at random, to a user.
Let users add one or more torrents, and seed.
Every 500 GB added/changed in the Internet Archive, make a new zip file, and torrent, and wait for some users to add that one. (Maybe needs a mechanism to ensure that users who have free space remember to check for new torrents.)
This seems like the simplest possible solution.
Note that some bittorrent trackers have more torrents of larger size than this, seeded healthily.