There is no hook for OnGUI that will give anything like a TextureID. It may be possible to use something to grab it out of the FBO… However!
The next release of Unity (3.4) should include the ability to prioritize scripts. So that if we have one script with a OnImageRender( ) that we want to capture the results from, we can now make sure that it is called before the second script, (The syphon script that includes it’s OnImageRender( )).
I don’t have access to the beta so I haven’t confirmed this, but am eagerly anticipating it’s release to verify that if our Syphon script calls the very last OnImageRender, that we should be able to include everything except for OnGUI stuff. (OnPostRender) doesn’t provide a TextureID and I’m not sure what is traditional done in this function, but I’m hoping it’s just script updating stuff like positions and AI, and not rendering.