If you're using pcvt, and you don't run X explicitly on vt0, then you
should be able to simply switch VTs to get to the debugger.  Perhaps
it doesn't switch modes correctly in DDB; I don't know.

Doing this reliably requires knowing many details about the video
hardware, which would rather defeat the point of pccons.  (As it stands
now, pccons runs on virtually any PC video adapter.)  I see no reason
pcvt can't do it, though.