So I got a TL866II+, and was able to program an AT28C256 ROM with the SCM; it's set up with an SIO/2 card and digital I/O module on the RC2014->KZ80 backplane. I used Alan Cox's RC2014 emulator as a framework for a basic serial-only KZ80 mainboard, so I'm relatively confident that the software works.
And... I get nothing. Time to debug. Note that on a healthy RC2014 in the default boot state, all 8 LEDs are lit up on the digital I/O board. That three are not lit might be indicative of something, I just don't know what.