For our divers we don't plan to add their gas rating to the c-card, just issue them a nitrox diver card, which means that they already have an open water or advanced OW diver card. The diver would just need to carry the nitrox card as the implication is clear that they are already certified as an OW or better diver.
Sadly, the industry rarely checks cards anyway. In 34 years of diving I have had to show a c-card at less than a dozen places, including going on dive boats, purchasing life support equipment (regulator and gas fills) including nitrox. The only place that I am sure to have to show one is in Florida State Parks, which is a credit to their diligence. The RSTC and the rest of the agencies should lean on the dive centers and operators to request c-cards as they ar supposed to.