It looks like the S/PDIF signal directly from the ATV to the Mac Mini is working under Windows as its signal shows up on the Cirrus Logic Digital Audio device in the recording tab of the Win7 sound app; same with the digital output from the Lynx to Mac Mini via USB - it shows signal on the USB record/Lynx Hilo device in the recording tab. However, there is no signal under the playback tab in the Win7 sound app for these sources.
It appears that the MC WDM driver does not intercept the recording signals, only the playback signals. So it looks like one does need a playback application for these sources to redirect to JRiver. I guess I could install a DAW application on the WiN7 partition and playback to MC. Question for JRiver: Is it possible to build in some kind of functionality in MC to intercept and playback/redirect the record signals without having another application do it?