zs0 at ...
 zstty0 at zs0 ...
 zstty1 at zs0 ...
 zs1 at ...
 kbd0 at zs1 ...
 mse0 at zs1 ...

  - does this mean I might possibly (with a bit of custom cablery) get to
 use the keyboard port as two extra serial ports? Obviously I'd have to
 tell the OpenBoot PROM to never for one second believe there's a keyboard
 plugged in and fail to use the serial console, but ISTR that's settable in
 the eeprom.

Yes, you should be able to do this.  

Eduardo