Nice! glad you are finding Unity + Syphon useable and useful 🙂
Brian Chasalow has committed some newer Unity 3D scripts and plugin to SVN. Are you using the public beta 2, or SVN versions of Syphon?
I mention because the new scripts Brian worked on have a slightly different rendering method, and I believe he has fixed some bugs with the Unity implementation.
In theory though, you ought to be able to get a texture reference from your post process pass, and send that to Syphon. I think that ought to be possible with Brians SVN version.