Isn't there a caveat somewhere in there that upper body restraint is only permitted on cars with a full roll cage? Lest the harness turn your spine into a supplementary b-pillar in a rollover accident.
Not exactly. Here is the rule:
3.3.1 Driver Restraints
Seat lap belts are required in all cars, and must be installed in cars with passive restraint systems that do not include a lap belt. Installation and the use of shoulder belts or harnesses is strongly recommended, how- ever non-factory upper body restraints may only be used in open cars, cars with targa-tops in the open position, or cars with T-tops in the open position when two conditions are met:
A. The roll structure must meet either the requirements of Appendix C or the Club Racing General Competition Rules (GCR) 9.4.
B. The top of the roll structure may not be below the top of the driver’s helmet when the driver is in the normal driving position.