The "good reason" you can't use undersized cables is because the OCP won't protect it in case of a fault. If there was a fault, the cable could burn up before the OCP trips. Allowances for bus taps are highly restrictive because of this.
However, if full ampacity (#3/0) cable is used, it's not a tap- in fact, the #3/0 can be considered a bus just as well as if it was a solid piece of bus-bar inside an enclosure. You can safely legally feed a bus-tap from the end of this cable, so even if you consider the polaris to be a bus-tap, it's OK because it's only a few inches long and meets all the other requirements of 240.21(B).
If you're going to feed it from 200A OCP, I'd recommend you use #3/0 and polaris lugs. It's safe, simple, and would be able to withstand fault conditions long enough for the breaker to open and protect it.
