Omega should probably split this off to another thread.
Well as soon as Mandate gets to the 1.0 beta phase I'm going to be documenting how to do all sorts of things to the engine like add new GUI panels, add new skills and commands, add new unit stats, add new object types aside from items and units and various other things.
Adding new commands takes editing a ton of files that you wouldn't expect for instance. There will still be things I didn't get done explaining for time reasons or I just forgot, but generally you can do anything useful following the documentations.
Some people argue that a good coder should just be able to read through the raw code and figure shit out but that's such a pain and I would argue not a good strategy to get new people into coding. First you hook them with easy to follow documentation, then when they are invested you force them to work with undocumented code.
Sadly for most people they only want to work with multiplayer capable engines RTS wise so nothing I do affects them. It might be good for you to write documentation for MegaGlest to help others and because its good practice though.