Epic Programmer Brandon "GreenMarine" Reinhart updated his .plan with word of a resource for C programming in Unreal Tournament, suitable not only for mod authors, but full-blown licensees:

If you are interested in C++ programming with the UT open source, please join the openut-development list at sourceforge. We are getting into some nitty gritty details that will definitely help programmers get to know the Unreal engine.

In fact, licensees might be interested in joining as well.

You can find the list signup at openut.sourceforge.net on the root project page.