Every now and then, there is a discussion of a GUI for GL. A rare and free gem, that might be made to work for a custom GUI is skia:
Is is quite fat (~1 MB stripped binary for a simple bitmap fill), and it has bugs. But it also has GL shader and fixed pipeline support (but in my view with too many bugs) to be usable yet with GL. Still, one could sw render a bitmap, then plaster a quad with it. It has some rudimentary controls and supports lots of platforms. Have you played with this library yet? IMO, it shows a lot of promise.