Well I cannot believe that I am the first person on the planet to ask this question, so I splashed out £60 on a 512Gb Integral Ultima Pro card just to prove a point.
As I suspected, it all just works, 512Gb seen by the RN7P and no issues writing to it. In fact, this particular card does sustained writes at 70MBps, which is pretty good at this price point. BTW, that's real world performance as measured by Windows drag and drop of a 100Gb+ Mp3 collection, via a USB-3 card reader.