I believe that it can be padded sufficiently with SFI 45.1 padding, that you will be well protected.;
View Roll Bar Padding \\ Roll Bar Padding at http://www.apexperformance.net
The SFI does not just rate this padding to endure multiple impacts from helmets. It's also to protect flailing appendages such as arms, legs, hands and feet.
I reject the notion that the car can't be a hybrid (Racecar that's streetable and street driven).. The originals did not have rollcages, but they also had steel spiders and their rear bulkheads were integral with the tub. That's a lot more protection than a fiberglass spider and square steel (in the case of most replicas) rear bulkhead support system.
I'm building a cage for mine - but even if it were not to be used for occasional track and instruction usage, I would still be building the cage..