Well, for once, If I don't use custom fields I don't have to retype the title of the piece (once for the cover and again for the actual header of the score). This avoids spelling mistakes, etc. But I believe I tried something like \fromproperty before and got some other errors too. I need to test this a little bit further to see If I can recreate the problem. Right now I can't do it, but I'll get back on that later.