If you're going to go through the trouble it would take to remove the
inner roof lining to gain access to the underside of the aluminum skin
to mount the magnet, you might as well just drill a hole and mount the
antenna directly.

I suppose. I used this trick once, but it was pretty easy to get to the
underside, and the owner was adimant about using a mag mount.

They are more forgiving about hitting low obstructions. A mag will fall
over, but if I snag my super-gainer, it will peel my roof like a grape.