With regards to JRiver, the key question is: is there a way to configure how JRSS downmix the channels? If so, we could ask JRSS to keep Center/Sub for Zone1 and to keep FrontL/FrontR for Zone2....
The Parametric EQ can do this.
The problem is that the Output Channels will still be "wrong". In order to have MC recognize that the "device" (a virtual device) is 6-channel, you'd have to set Output Channels to 5.1. You almost could:
1. Make a Zone for each DAC.
2. Set Output Channels for Each to 5.1.
3. Set Parametric EQ to delete the channels not needed for a particular DAC (delete everything but the front mains for one, everything but the sub and center for another, etc), and remap them to as needed to the front main channels.
4. Link the three Zones.
But, the issue is that none of the DACs would work right, because they'd still be receiving a 6-channel source (just with 4 blank channels).
In order to do it, you'd need an actual virtual "sound card" that you could set as 5.1, and then route IT to the individual DAC Zones (each of which would be set to the proper 2 channels). This functionality isn't built into MC.
But, it isn't built into MC because it wouldn't work very well. The Linked Zones in MC are very low latency, but the system is designed to be able to link multiple zones even over the network (and all sorts of other considerations), it isn't designed to be ZERO latency (this isn't possible). Even if you managed to get it incredibly close, the different DACs are very likely to be out of phase with each other.
Ergo, you wouldn't be helping sound quality, you'd be hurting it, compared to what you could get with HDMI output and a decent receiver in 8-channel PCM mode.