PCA Potomac might accept you. These are their rules:
All open top cars (e.g., cabriolets) and all cars that exceed the modifications permitted for the “Stock“, “Prepared”, and “Spec” classes per PCA club race
rules (i.e., would be classified “Super Classes”) must have roll bars that meet PCA club race specs for materials, design, installation, and height. Top of the main hoop TF must be at least two inches above the driver’s/passenger’s helmet when seated in the normal driving position. (Details in Appendix A of Club Race rules.) Integral roll
bar in the Boxster and pop-up roll bar in the 996 cabriolet are acceptable. 914’s, Targas, and Boxsters with hard top in place do not require roll bars. (Note: Roll bars in cars not requiring roll bars are not subject to the above requirements, but must be firmly secured to the chassis.)
If they consider the 996's pop-up roll bar as acceptable, they might accept your car (I assume that the E93 has a pop-up roll bar in addition to the retractable hard top). They have an event at Summit Point August 13-15, with free tech inspection one week before. See pcapotomac.org for more information.