Thats an interesting idea. Honest answer is, I have no idea.
I suspect that Parallels is shimming the graphics drivers somehow, but I don’t know enough about how it works. Secondly, its closed source as far as I am aware, so they would have to add support for Syphon.
I’m going to be *very* surprised if the engineering work would be worth it for them. I would ask on the Parallels user support forum and make a feature request. We don’t have any real way to shim into their app.