Yes, there is a very good UPnP-Client for Mac OS-X. It is called XBMC and can be found at xbmc.org
Mediacloud is only a video player and (as far as I know) does not play music files. Boxee does not work at all on my mac and it is only alpha software.
New (expensive) idea:
How about buying a second NAS? Both NASs work with an own Twonkymedia and so each Twonkymedia doesn't see the music files inside the other NAS.
Is there any problem with two Twonkymedia servers inside one network?
Normally I want to use a lossless codec like FLAC to store my music on my NAS (twonky 4.4.6). But one of my streaming clients (Philips MCI500) does not understand FLAC :( So I thought I could use DBpoweramp together with its MultiEncoder and rip each track simultaneously with codecs FLAC and MP3. :i...