Well I've finally been able to recompile a current kernel with
retinaspeedhack enabled and using custom screenmodes and it ALMOST runs
perfectly, "except for a few things" (tm):

Have any of you RetinaZ3 users noticed an ugly black square taking the
place of the cursor bitmap when you move the mouse to the top of the
screen?

Have any of you RetinaZ3 users had any problems when using xman? (black
bars taking the place of text)

Finally, I can't seem to specify what screenmode I want to use in X11R6.
As I understand it, I should put in the xserverrc:

X -mode <whatever>

However, no matter what mode I try, it just runs the default.