A copyright protection feature of iTunes prevents you from syncing a remote library to your iPod, creating playlists from tracks on a remote library, etc.
You can play music / playlists on a remote library but that’s it.
iTunes see’s Firefly as a remote library.
One of your PC’s will have to access the music stored on the NAS directly. This PC can be used to sync your iPods, create playlists, etc…
You could also try the Java Firefly client found in the Add-On Software section.
This little application can be used to create playlists. It’s still under development but it gets the job done.
It can’t sync your iPod however.