Gee it's quite strange reading about "one design" regattas in the states.
In Australia if any regatta is restricted to only one design racing it's called a state heat or a national title. Apart from those, every other regatta that I know of is open to any and all classes (including all monohulls as well). There is such a variety of different designs sailing, and spread out at most clubs around Australia that if "one design" was ever required by any of the clubs, they would be reducing the participation numbers at any one regatta to such a degree that it probably wouldn't even pay them to run the event, (and the clubs supply the club rooms, vehicles to tow the boats to and from the beach, showers, bar, meals, rescue and duty boats, flag officers, they set out the course and they time the competitors as well as calculate the results, hear any protests, and give out the trophies). Then again, all regattas in Australia are organised and run by the various yacht clubs and not by any class of cat/monohull, so it is in the interests of the clubs to have as many boats as possible at every event. The races are always “open”, but usually divided into “divisions” each with their own start. IE, 18’, 20’ cats (and anything in between) start and race together, any cat greater than 14’ and less than 18’, start and race together, and all 14’ and less similarly. (Although the division break up is now generally not divided by the size of cat but more by a yardstick/rating similarity division). If the numbers of any individual class (whether cat or monohull) is great enough then they may at times request their own start, but they still race out on the water with all the other classes present. This situation has been the same for at least the last 50 years and it has worked (and still works) to perfection. Don’t you guys have similar clubs there, where, as members of those clubs, you have regular “open” weekly club races as well as ‘open” regattas run by the respective clubs?