Good luck with all this, and I hope the site doesn't have to go into exile to somewhere outside the American Empire.(Which includes England, where I'm writing this) I've just been checking the National Music Publisher's Association website. Even aside from the issues of principle, it really is bloody useless as a way of finding any interesting songs, and extremely irritatating to find your way round in it.
You're not alone in this, as of course you already know - I came across a lovely site with lyrics of French chansons, http://www.math.umn.edu/~foursov/chansons/index.html, and it's had to give up including some of the best chansonniers around (including Jacques Brel), for just these kind of reasons. But of course those were songs which were definitely written by identifiable people, and most of the songs on Mudcat/DT are either traditional, or written by people who would be proud to be seen as of the tradition, and thus included in the hall of honour.
(In the unlikely event that you ever feel like including any of the songs I've written, you hereby have my irrevocable permission to do so.)
Kevin McGrath kevin.mcgrath@bigfoot.com www.bigfoot.com/~kevin.mcgrath