You are right:
ssh -X
transmits controls and not frames.

But this is exactly the reason why transmission of accelerated 3D graphics is POSSIBLE (e.g. aiglx).
openGL is NOT possible with VNC. Because VNC only transmits frames. But frames are useless for 3D Hardware acceleration.

openGL needs to have 3d objects (transmitted by controls, like postscript.)

ssh -X transmits 3D hardware controls with aiglx e.g.

My question is not if 3d graphics by ssh -X is possible.
I know that it is possible.

My question is if it is possible with the n900.

This depends on the implementation of SGX in the x-Server.
Is hardware acceleration supported in x-server?
Is it supported in a way that is compatible with transmission by ssh -X.
(e.g. GLX extension: texture_from_pixmap)