Cantabile actually does save both relative and absolute paths for referenced files (this includes media player playlists, and songs from set lists).
I think what’s catching you out is that it checks for the absolute path first and if the file isn’t found there is looks at the relative path. I should change this and I’ve made a task for it.
Also, the settings Tools -> Options -> Songs etc… only affect the default location of the File Save and File Open dialogs and aren’t used in resolving file locations at all.
TL;DR: if you move everything it should already work. If you copy everything it won’t, but I’ll fix it.
As for the bundle idea… cool idea, but way down on the list.