Chuck Westfall about Canon cameras and high trigger voltage strobes. He basically tells his Canon folks that hooking up such a flash to a Canon camera might even work but can mess up things in the long run. Don't do it, and if you have to, there are safe sync adapters which convert the high trigger voltage to one the camera is rated for.
I do agree that a camera like the RZ67, which was marketed as a professional studio camera, should support higher trigger voltages out of the box, but for whatever reason Mamiya thought this would not be necessary. BTW don't forget that this camera used to cost more than US$4000 and wasn't sold at Fry's Electronics, so one would assume that people buying it would either know or learn from Mamiya's sales people which flashes they could safely use with their camera.
And all this is completely besides the main point anyway: don't connect a flash with more than 50V trigger voltage to Yongnuo radio triggers, they are not built for that.