I guess two-way sync is something players like MB may have to think more about in the near future because of the proliferation of devices which will actually allow you to develop and edit part of your library on the device. Personally I'm still years away from buying a phone or tablet which has that capability.
For the foreseeable future I'll just be using a dumb, old-fashioned mp3 player - I send tracks to it, choose and an album or playlist and press play, so I'm only concerned with one-way sync. Yes, I know - so 'last decade'. You idea is a fully-fledged solution, whereas I just want a quick band-aid.
The two workflow options I seem to have are:
Maintain one sync playlist for the device, add a couple of new albums to it, hit 'sync' once a week, and wait for an hour while Gigabytes of files are overwritten.
OR
Maintain two playlists - one for an occasional 'proper' sync using the sync button, and another called 'new albums to manually move'. I'd put new albums on both lists, but once a week I would manually drag albums to the device from the second playlist, and then manually delete them from the playlist, hence avoiding a full sync.
Bleh!