> The diag switch makes the console a 9600 7E1 on ttya.  However, this
> console starts doing things at the same time that the video would.

Not.  At least on the 3/50 and 3/60, setting DIAG causes ttya (and I
think ttyb also) to report progress through the selftest.  Once selftest
is complete, the invitation to press a key for extended tests appears
on all three ports and whichever one you respond on (if any) becomes
the console for the duration, regardless of the selection in the eeprom.