Part 1 (easy make it.. probably): I think that a header that already include all GLtypes would be very usefull (when you have classes that passes around GL objects so you can include in the header only the GLTypes header, and in implementation files you include the generated file of glLoadGen. (in that case I would just add an option "-ExtractTypes", the generated files will try to include types only if not already included). This is good because you can rework implementation files keeping recompilation times very low. Also you don't expose zillion of things when including classes.
Part 2 (hell to implement): Need finer granularity: What about adding incremental headers, Assume I build my library with GL 3.3 core, then someone want to extend it with GL 4.3 functionalities: "-from=3.3 -to=4.3", it generate an header that include the pre-existing 3.3 header wich may optionally include the GLTypes header