I finally managed to remove opengl from moonfltk, making it compatible with fltk-1.3 instead of the bloated fltk-full. I've uploaded the update and it should be merged probably next Sunday?
I'm not a big fan of lua myself, but I like it well enough. I've also seen some active users who seem to have some experience with lua.
Being as light as micropython and as fast as C, I wonder if it wouldn't be appropriate to integrate lua into TinyCore? With moonfltk, it opens up the possibility of completely replacing Xprogs with easily “updatable” scripts instead of the delicate low-level programs (C, c++).