iTunes, like many pieces of software from many manufacturers was never intended to be used in a shared-storage environment. Instead the intention is that you back-up to external storage, but you leave the default storage as is.
Now, that said... what you'll need to do is force iTunes to believe it is using local storage (with the obvious disclaimer that this is at your own risk).
On OS X, what happens is that each new user created on the system has its own directory under the /Users directory. Apple then follow a standard path of ~/whatever for key folders such as Pictures, Music, Library etc.
So in your case, what you'll need to do is to enumerate the locations where iTunes stores its data and then symlink the local directories to the NAS directories.... i.e. ln -s source destination
Of course if you had a separate OS X server box then your life would probably be a little easier as I believe OS X server allows for remote home directories.