VR desktop = GLX_EXT_texture_from_pixmap + ovrsdk
Quick experiment showing a very smooth framerate as a result of GLX_EXT_texture_from_pixmap, no host to device copy necessary, application framebuffers are mapped to textures by the device driver.
Set XCompositeRedirectWindow on the window to CompositeRedirectAutomatic or CompositeRedirectManual, as desired.
Get a GLXFBConfig via glXChooseFBConfig
Get a Pixmap (offscreen storage) via XCompositeNameWindowPixmap
Create a GLX pixmap via glXCreatePixmap
Finally bind the framebuffer to the texture via glXBindTexImageEXT (Note: you may specify LEFT or RIGHT buffers if the framebuffer is GL_STEREO)