I've been messing around with saving playlists lately (as m3u files), mainly to use them on a tablet with the Android foobar2000. I'm not sure I've mastered that one yet, but when you save a playlist on a PC at least, fb2k only seems to save as much of the path as necessary.
As an example, if you have an MP3 buried a few subfolders down, such as
and you save the playlist somewhere outside that folder structure, fb2k saves the full file path.
If you save a playlist to FolderA, it'll be saved in the playlist file as
or if you save it to FolderB
The playlists need to be located in the same place in the folder structure on the second PC when you import them, or fb2k starts looking from the top. Can't find "F:\FolderB\FolderC\Song.mp3" etc.