What you need, maybe more than developer or filter considerations, is fill flash. Do you have a decent flash for the Leica? I believe that the Leica is a leaf shutter camera, and syncs at all speeds?
I've had good luck, shooting with my leaf shutter TLRs, shooting on bright sunny days. Let's say that I meter a scene at f11 @ 1/125. If I set the camera at f11 @ 1/250, I'm underexposing the ambient exposure by one stop. Then, if I set my auto flash ISO speed at one stop faster than the film rating, I'm underexposing the flash by one stop. That's a pretty good combination, but you really need to experiment. Underexposing by 2 stops on ambient and one stop on flash will give you a very subdued background, but your main subject should still be nicely exposed. Again: experiment.