From the "What's the Facts" section:

"Yes, but with a few caveats. Some tracks are more restrictive than others so check individual event pages, but generally, if your convertible is actually a Targa or T-Top, no problem.

If it is a true convertible it needs to meet one of the following criteria:

• The convertible vehicle is a 2006-year model or newer, does not have forced induction, is less than 8 cylinders and has not been modified meaningfully beyond factory performance potential. (Examples: Engine swaps, aftermarket forced induction.)
   
• The convertible vehicle is equipped with documented, factory-installed rollover protection (Examples of manufacturer documented rollover protection would include hydroformed and reinforced a-pillars or windshield frames, and factory-installed roll bars and/or "pop-up" bars that are designated as rollover protection.)
   
• The convertible vehicle is equipped with an aftermarket roll bad that meets or exceeds standards set in the SCCA Time Trials Rules. "