You're not the only one that would like to be able to install MC on a nice Synology! I too have PlugPlayer on an iPad but would prefer small NAS unit for media serving instead of having a power-hungry PC on all the time.
Note that the Synology OS does have a built-in DLNA server, but it is nowhere near as good as JRiver MC. It has the usual inflexible tree organisation of Album, Artist, Genre and Date, which is absolutely useless for any comprehensive media collection. The Beta version of the next OS has just been released and it does in fact add Album Artist to the list of available tags for organisation, which makes it usable now, but there's still a long way to go to catch up with JRiver standards. It depends how you've tagged your FLAC files and what sort of cataloguing you want, maybe the Synology DLNA Server would work for you.
Squeeze Server can be installed on Synology units and I believe Twonky also runs, but once again neither of these comes anywhere near to JRiver.