True, you cannot set the ISO as such. However, there are two subterfuges. First, mask the DX contacts on the film cartridge to get ISO 100, or tape an accessory DX label to get your desired ISO. Second, apply an exposure compensation value of ±2 EV in increments of 1/2 or 1/3 which will persist until you change it (even across power cycles).
Rico Tudor. Leica M4, IIIb, 28, 35, 50, 90, 135, 280. Contax T, RTS; Canon; Nikon; Sony; Profoto