Never mind, all. Upon further inspection I don't believe this to be a JBoss issue but instead an issue with the third-party library.
The third-party jar uses its own custom ClassLoader that I need to fiddle with instead.
So if anyone knows how to get GATE ( http://gate.ac.uk ) to play well with JBoss, help me over on the GATE mailing list! Thanks.
You should be able to toss everything into a .sar file (your classes, third party stuff) ... If you specify a classloader name, go to the JMX console and find it, and test what classes you can access.