Does anyone know if you can use the Sony Wireless WRT8P/4244 plug-on transmitter with the Wireless UWPV1/4244 set?

I can't specifically answer your question for the Sony, but the Audio-Technica 1800 camera-mount system I just received (plug-on and belt-pack) is described in the operating manual as being compatible with the Audio-Technica 3000 base-station systems (which I also have but NOT in the right frequency band!).
So it is possible with some brands.
The only consideration in the AT case is the 1800 selects frequencies in 25kHz steps while the 3000 uses fewer frequencies in 125kHz steps, so they would have to be set to an exact match.