Torrents would work perfectly well, and are in fact one of the proposed solutions. The problems you mention are real though. One idea is to use a custom BitTor

Torrents would work perfectly well, and are in fact one of the proposed solutions. The problems you mention are real though. One idea is to use a custom BitTorrent client which the user can install (a bit like the ArchiveTeam Warrior VM). The user would tell it how much disk space they want to donate, and the client would join swarms over time to fill up that donated space. The user could even join specific swarms representing content that they enjoy and particularly want to save, but wouldn't have to go to the hassle of picking every torrent manually. Also, IA already provides a torrent for every single item in the archive with IA as a web seed, so this could be pretty simple to do from their perspective.