"No reason Dropbox couldn't figure that out and have one machine download the file and then distribute over LAN."

They could have each machine download a chunk and have them swap chunks locally. That would asymptote in time towards a single download.

That would handle cases of the "one" machine going offline close to the end, or having it being the slowest machine available.

In other words, a slightly more managed Bit Torrent.

That's a great idea to handle those sorts of edge cases.