Though
http://www.ootp-leagues.com/ is supposed to do what you are talking about, I don't think it does it as effectively as I think people would like to see. There's many problems with it imo:
-there's far too many leagues on it
-(I assume) some leagues are defunct (no longer exist)
-I doubt many commishes update changes to their league
-it does not list some info that I think people would like to see
-I doubt many people who are looking to join a league either know about it or figure it's worth the time to dig through
-it would be great if there was some sort of rating system of the leagues
I think a better idea might be to ask .400 to make some improvements to that site and promote it more.
I do like your idea about a place for hopeful GMs to post what types of leagues they would like to join. Though I'm sure someone might say that is what this board is for, I think making a site like the above leagues board but for GMs would be more efficient. It would probably be best to put it on the same site, just have its own section.