If all playlists were sent together as a unit near the end, then cancelling there would not be a big deal -- I expect that the time required to send ALL playlists will be less than the time for a single (typical) MP3 file upload.. so maybe just never cancel in the middle of doing playlists. For serial, this wouldn't be as pleasant as it could be, but for ethernet/USB it should be no issue.

Cheers