Hopefully with Wayland coming around, this will all be simpler. The reference Wayland compositor (Weston) has a built in fullscreen zoom feature which is basically identical to OS X's (hold down key, scroll).

In fact, the default implementation in Weston is, better in my opinion, and also enabled by default. Instead of having to go to the edge of the screen to move the region, it moves the region with the cursor in the same position relative to the screen as the region. Of course, for those with dyslexia or some other psychovisual disability; it may be worthwhile to have an option to go the OS X route.

Thanks for the input, I'll maybe put it on the GNOME bugzilla if it isn't already there. :-)