We are all waiting for Syphon for PC.. In the meanwhile, if you have a powerful machine with three outputs, AND you don't care much about resolution AND like tricky setups you can experiment:
on the first monitor you can have your quartz-composer like program (perhaps the good old visualjockey), you capture this screen with Hmelioff labs VH multi camera studio capture module (note: no hardware accel here), on the second monitor you can have resolume GUI.
The captured stream is seen as a webcam so you can pipe it into resolume, and finally on the third monitor you can have your output.
If you need an extra non-accelerated screen for another GUI you can pump a virtual screen out to an old laptop via network using Maxivista.
Weird? Yup, a little.. Stable? I am still testing (with two other programs, no resolume for the moment) and when properly set up it never fails.
Drawbacks? the main one is that it's not possible to capture an accelerated/overlay output.. technically it can be done (fraps rules!) but strange enough I didn't find this feature in ordinary capture software.
)