This is just a thought...

It seems that there are many problems getting internal video to work on
the PB1x0 series, and I had an idea last night... if you call
GetWmgrPort(&g) you get a GrafPtr to the entire screen in g, so would
g->portBits.baseAddr (I hope I spelled it right) be any help in figuring
out the video/memory mapping? There is also an element of g->portBits that
gives the length of the video memory. I can't remember what it's called
right now.

-Jonoddie