or does geeXLab need windows and cannot display directly onto the framebuffer?
GeeXLab for RPi exists in two versions: one with OpenGL 2.1 and one with OpenGL ES 2.0. GeeXLab with OpenGL 2.1 requires X11. GeeXLab for OpenGL ES 2.0 does not use a window manager, it renders the scene directly in the framebuffer. Maybe you can try this version, and start GeeXLab and load scenes via command line.
In the code sample pack, you will find OpenGL ES 2.0 demos in the gles-20/ folder.