In vintage racing the rules are a little softer around roll cages. Mine is more than is required. I sent the pictures of the roll cage to SVRA (that's where the logbook is) and they liked it. I have raced the car in SCCA vintage races and have not had any complaints. Just reviewed the current NASA rules and could no see anything in the roll cage section that addressed open cars. The rules were designed around a car with a roof. Have you looked at the designs of the roll cage manufacturers that they reference?
My driver's side continues up to the hoop to provide two support points going forward.
Morris, on this site, raced a Kirkham 427 car in SCCA for many years. You could look at his design. It was raced in GT1, which is a very competitive race group.
You should also check with Factory Five about the possibility of using their roll cage for the challenge car series that runs with NASA. It may be possible to have them fit the roll cage to an USSRC/FIA car.
Jim