Not very discreet but I've mounted my GoPro using a GoPro suction cup mount in various ways: On the roof, rear window (outside & inside), front windscreen (inside) and also on the door panel.
When I went to Castle Combe for a track day they were ok with me mounting it to the inside of the front windscreen (does vary between circuits & organizers though I know).
On the rear window facing forwards looking over the roof.