I believe Rotondo and Kompressor help as well. However, I believe that Kompressor only works on things that have to do with editing/changing the main engine. Rotondo basically performs the same tasks as Badspot does.
I don't believe that Rotondo has access to the source code. Look at the bots he made - they use hacks to get things (like the GUI) to work rather than being tied into the game, as, in my opinion, they should have been.
Edit: To add on this, Badspot seems to do more work in TorqueScript than in C++, while kompressor handles the more advanced issues such as the oct-tree, networking (?), and shaders.