It works!! I've tested it using the MIR Pro Trial with the 'start venue' and the Mozart Saal trial venue. I am using Cubase 6.0.5 on a PC. The VE Audio Plug-Ins are in FX channels that throw 'instrument groups' into the multiple MIR Pros, which all run on one DAW.
So this is the final (working) diagram:
---> DAW VE VST 1 ---> Slave 1 VE Host ==> DAW VE VST 1 ==> DAW VE 1 ==> MIR 1
DAW DAW ==> VE Audio Plug-in ==> DAW VE 2 ==> MIR 2 ==> DAW
---> DAW VE VST 2 ---> Slave 2 VE Host ==> DAW VE VST 2 ==> DAW VE 3 ==> MIR 3
Thanks a lot, Karel and Dietz!
I'll post quantitative results when I have them. But for now, with 3 MIR Pros running and five audio input channels each, all playing, my 2600K system has an 18% CPU load in Task Manager, for the whole system that is. Not bad at all, and much less load then I expected. This really sounds promising.