Dan, any chance to convince you to give it simply a try reproducing the issue? All that you need to do would be adding a fresh user and adding a line
DisplaySize 16 16
to monitor section of your xorg.conf. This should at least break completely
the font sizes for the (Gnome) Xsession of the new user. Not sure if this
will also break gdm itself, but I think so.
Dan, any chance to convince you to give it simply a try reproducing the issue? All that you need to do would be adding a fresh user and adding a line
DisplaySize 16 16
to monitor section of your xorg.conf. This should at least break completely
the font sizes for the (Gnome) Xsession of the new user. Not sure if this
will also break gdm itself, but I think so.