Well, final word is in. 
I got to talk to Jon Z. for a spell about this issue, and it turns out it's not totally unknown that the DZ-2500 gets testy about talking to the CSM2. Jon talked to Dennis Zander about it, but there was no fix forthcoming. The speculation is that for some reason when powering up the DZ-2500 sees something on the serial input that makes it think it's in the wired remote control mode and it stops listening to TMCC commands. That would explain why all of them stopped listening to the serial data and not just some of them.
In any case, I think I may have one more card up my sleeve. I suspect that delaying the power being applied to the DZ-2500 switch machines for a short time (a second or two) would likely allow any serial data stuff to settle and not to confuse the DZ-2500 mode selection. I may try that before I throw in the towel totally on the CSM2 serial data.
