This stuff is / will be used for sure!
My project uses FLTK for GUI, but it is OpenGL centered software. The new version will need OpenGL 3.2 or higher (the first to support geometry shaders and still can be implemented without hardware acceleration). Just now I am debugging the shaders. It is difficult to me to instruct GPU to draw natively non-Euclidean geometry, not only some model thereof.
I tried your FLTK-haiku (1.3.x branch) and observed the lack of OpenGL context. I am oscillating between adding OpenGL to Haiku version of FLTK (it would be nice to upstream Haiku port to official FLTK project, which is really crossplatform) and writing native Haiku API user interface (I always wanted each GeomSpace version to have native look and feel).
The later option mean much larger effort, because the GUI related code is already about half of the codebase. I would be pity to duplicate this code for each and every OS (essentially different 3, Haiku would be the 4th). This GUI code is rather simple and not interesting. OpenGL code instead is what I want to showcase. If FLTK would support (officially or inoffilially) modern OpenGL, it would be brilliant for further research of GPU capabilities for native non-Euclidean geometry (beside the geometry itself of course).