>mcd caused Really Bad JuJu.

Of course it did. The stupid "mcd_find()" routine unconditionally
claims a match .  Pre-port-allocation, that's a total lose.
It would clobber any and all of the ISA devices at 0x300.
"Really Bad Juju" is an understatement :).

With both bus_space_map port allocation, and nuking the mcd
promiscuous match, and probing *after* all other drivers at 0x300, it
should be a different story.

If I find time (famous last words) i might build a boot floppy with
mcd support and try it on as many machines as I can find.