A 1971 Plymouth Roadrunner rear licence plate bracket is spring hinged to allow access to the gas filler benind the plate.
Not sure how readily avaiable the old Roadrunner brackets are, but I am sure other manufacturers have similar ones available that would work as well.
epper:
My mother drove a 1968 GTO and I remember that car had the hinged license plate over the gas cap. I think a lot of GM cars of that era did.