What a lot of folks were doing was just setting up ruler guides at roughly the place on thier 12 x 20 artboards that were “Out of Bounds” You can get fairly close. Then keep your design inside those guides, and it shows up the same way on the screen in the GFUI.
The guides do not get pulled over into the GFUI. But the disadvantage is you have to set them every time…no way to save guides. (I don’t think there is…I was wanting to see if I could write an action for it, but I always get busy on other things.)