I like the idea of using the archive's torrent as there would be a permanent seed.
If that functions as I would expect a "web seed" to, you won't see the seed until you actually start the download, so it will look like a dead torrent on bt's site. Cyanogenmod uses a similar system for their nightly releases.
But it will help distribute load, which is really the point. Torrents are designed to ease distribution in a timely fashion, not provide a permanent/semi-permanent method of access.
I'm not sure how to work through the details...
try taking the archive.org torrent file and adding the bt.etree.org tracker as an alt tracker (but keep the hash data from the archive set and the archive torrent tracker), then post it on bt. I think that's the best hope for "cross seeding" in a single torrent file.