I've had this same desire recently, when thinking of the problem that I cannot adjust any controls on my amp from the drivers seat without cabling it to my lap.
Also, its a pretty amp I wouldn't mind it not being hidden in the trunk.
The way the roof of my car (most cars?) is curved, I'd imagine one would only see the amp if they crouched down and looked through the bottom of the front windshield.
I have a sunroof, and I would want to mount my amp on the rear half of the roof. I haven't ripped off the headliner yet (I was thinking of hitting a salvage yard to see if I can look at the roof of a similar car with the headliner missing).
So far my only idea was to get a large plastic plate (non-bendable), and screw the amp to this plate through the headliner. The plate would distribute the weight of the plate & amp over however big I want the plate. I have no idea how feasable this would be, in terms of holding the amp up without bouncing around, or as a theft deterrant (if some scoundral could just yank on the amp and take my headliner with it, i'd opt for a sturdier solution).