The 37mm rb fisheye is excellent and will work on the rz. Way more than 90 degrees!!! But if you crop down to 90 you'll not be disappointed! An especially good solution if you are open to electronic corrections later.
Let's see, the rb 50mm on the rb with 6x8 back might get you close to 90 on the vertical, probably just short.
The mamiya 645 28/4.5 will get you past 90 and I suppose that you could mount it on a pro or similar with waist level. Very pricey lens, that.
I guess you could scale focus a crown graphic with a 90mm lens, using either 4x5 film or a 6x12 rollfilm back.