You should be able to connect the NAS to the time capsule via ethernet, and share it to your computers.
I would recommend hard wired between the TC and your computer for the initial file transfer, but for general playback, even a wireless-G connection is fine.
I use a remote box for all my music storage (A linux server box, but for this, basically the same as your nas). and it works fine. I also have a mini hooked to the TV, and all i have to do is copy the library file from my laptop to the mini, and everything works.
One hint that i found, if you have the "iTunes music folder location" point to a non-standard location such as remote file share, and iTunes can't find that location on start up (it only looks once), it will automatically reset to the standard "user/Music/iTunes/iTunes Music". So for some reason if you start iTunes without a network connection, it will automatically reset the location.
If you replace the "user/Music/iTunes/iTunes Music" folder with an alias to your network storage location and name it "iTunes Music" it will default to the network location. You still won't have your music without network, but at least it won't change any settings. and everything will be good once you get reconnected.