I strongly agree that we should allow either an offset bushing or slotting of the upper control arm. The variation between cars is too great, and it's unreasonable to expect the average guy to have the time or resources to dig through a pile of junkyard parts until he finds a combination that gives enough adjustment. I've seen shop-built cars with a TON of rear camber and I've seen cars that could barely hit -3 deg.
Slotted arms are easy enough to find in tech, but I can think of several ways to increase rear camber that would be virtually undetectable. If your car happens to be one that can't hit optimal camber, you are at a disadvantage unless you are willing and able to either (1) do the parts dance or (2) intentionally modify something. Either option might be simple for a big shop, but not for a one-man band.
I also strongly agree that teching camber is a terrible idea. Whether from measurement error or actual movement, my camber changes by .1-.3 degrees every time I check it. I know camber is a tech item in other classes, but I'd really rather not go down that road with SM. Chasing variation in scales from track to track and day to day is bad enough. Let's not make it worse.
Just like we did in the front, we should give every car enough adjustment to go beyond optimal and call it a day.
And while I agree with Danny's DQ, I do NOT agree that it was on par with an oversize restrictor or Sammy's shocks. At least to my eyes, Danny's rear camber was well within normal Spec Miata range. The method for achieving that camber might have been technically illegal, but it did not result in a performance advantage. It simply brought the car into the same adjustment range as everyone else.