But can’t find it in the ndk version 9 gl2ext header. Is there anything else I have to do to make this run on supported Android devices, or should I just forget it?
I think you can just add the necessary function pointers to your code even if the NDK headers does not directly supports it. I’m not familiar with GLES extension loading, though I suppose it should be pretty same to how it is done in WGL/GLX.
It is another question whether any Android devices support EXT_occlusion_query_boolean so I would definitely choose to have also a fall-back path.