I don't think the RB and RZ finders are interchangeable.
Actually, to the best of my knowledge, they are. Of course you will lose AE metering on the RZ if you had an AE prism finder on it. The camera label is on the finder, usually. That means your RZ will look as if it is an RB!
Thank you to everyone for your replies. So I bought an RB67 waist-level finder and tried putting it on my RZ67 body (my camera came with a Prism Finder that is metered, and I wanted another option because there may be times when I want more portability. Anyway, the finder didn't fit. There are no buttons on the RB finder like on the RZ finder, so you would have to do some filing down of either the pins or the tabs that slide into the rear 2 slots, and this would make it fit too loose. So, I bought a used RZ finder and am waiting for that to come. By the way, I really love the aesthetics of the waist-level finder. I think it is beautiful in it's simplicity!