I have added a few 78s from archive.org; my approach was to add the 78 itself as a single, with a “stream for free” link to archive.org. See https://musicbrainz.org/release/ee039d99-39a6-4215-afef-fa23fb28a39e (where I seem to have only added a stream link to “I Wonder…” ) for example.
Some time back I started a discussion about informally traded recordings here, general consensus seemed to be that “bootleg” was the best fit. I haven’t actually added many, though.