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 (or the one most needing seeds), 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 torrents that sum to a larger total size than this, seeded healthily. Their torrents tend to be smaller than 500 gb though.
The Geocities torrent, at 900 gb, was an exceedingly large torrent, and there was some trouble keeping it seeded.
At 500 GB, this leaves out users who have some smaller fraction of a disk available to donate. This might reduce contributors significantly. A smaller chunk size might be better.
The user needs to keep their torrent client running, or they won't be counted as a seed. Offline or rarely online storage can be used, but won't be counted. So counting seeds will undercounf the number of copies.