The biggest problem with exposed luggage is that you want to protect it from rain, dust, dirt, insects etc. So would need to be enclosed in plastic(garbage bags etc) or use one of those plastic pod things or make similiar.
If it does block the high-level brake light, you could wire in a socket at the back of the cockpit, even on a lead under the seat and plug in another high-level brake light that attaches to luggage rack.
Don't know what you guys are complaining about with luggage space, with GT40 you need to bring friends with plenty of boot space.
