> Try to remove the GFX card. This should force the machine to serial console.

actually, the prom goes headless if it doesn't find a keyboard *and* the
nogfxkbd environment variable is unset or 0 (or, if the console variable
is set to d/d1/d2 -- the g setting attempts to find a keyboard, and falls
back to d otherwise).

that particular prom variable came into existence when the prom couldn't
detect a keyboard at the other end of the 75' cable kit that was desirable
back in the days of the big 4D deskside and rack systems (trust me, you
don't want to sit next to a 4D440VGXT :)