You mentioned that ProPresenter can run QC comps (albeit, not natively loading QC Plugins). This leads me to believe it is using the Quicktime layer to read QC comps.
Modul8 does something similar, but with a 3rd party, non official, “skank” QCPlugin named QCRehab you can actually get Modul8 (and other apps) to load 3rd party plugins.
Try installing QC Rehab, and making a QC Comp that loads a Syphon Client and listens for a specific named server, which you could run from another QC Comp.
I’ll bet you it works.
Let us know.