Mamiya did a prototype 43mm aspherical lens for the RZ, but it was never released. This was in about 2002 or 2006 I think. It was shown at photokina.

Adapt Pentax lens: Not feasible for several reasons.
Adapt LF lens: Not feasible for several reasons (Mostly back-focus distance).
Adapt RB 6x8 back: Not feasible. Only adds 4mm of image width anyway.

Your best bet would be get a Mamiya 7 with the 43mm for the wide needs.

That said the 50ULD is a great lens, quite wide on RZ and should suffice generally. Focal lengths are not directly comparable to 35mm since the format aspect ratio is different. The 37mm fisheye is a nice lens too, not as fishy (barrel distortion) as many 35mm fisheyes for some reason.

-Ed