I've been thinking about a way to integrate a Nuvo Grand Concerto (or maybe Russound CAV6.6) with the MSC-400/MX-900. Specifically, I was wondering if I can use 2-way RS-232 communication in a way that would allow the MSC-400 to control the sources for the GC and use variables to synchronize which GC zones / sources are on and off etc... Has anyone done this? It looks to me like the MSC-400 doesn't accept responses from devices, and can only send 1-way. Am I missing something?

The 400 will receive RS232 feedback from the devices, since in testing, you can view responses, but I believe it just disregards it operation-wise, as it it meant to be a one-way RS232 communication device only. The MSC-500 is a similar product that has 2-way 232, but is still not actually available.