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