Here's the section from the GCR 17.27:
"2.CLOSED SPORTS RACING CARS,PRODUCTION CARS,
IMPROVED TOURING AND GT CARS --In front of the windshieldon either the cowl or on top of the fender, but close enough to the windshield to be accessible if the car is overturned. Alternatively, it may be mounted below the center of the rear window or on a bracket welded,clamped or bolted to the roll cage or dash, easily accessible through the open window. (Drilling of holes in roll cage to attach the bracket is prohibited.)"
It doesn't say accessible by the driver, only accessible, and here, since it talks about it being outside, it clearly means accesible by corner workers or rescue folks.
Driver access is an added nicety, but not required.