To give you an idea with I use with my Nikon FA and FE and my Mamiya 645 Pro is older Canon potato masher flashes (533G and 577G). I haven't really had any issues with going with the flash driven f-stop for auto. The only exception to this was some family pictures I did with Porta 160 about a year or so in full sun, I should have adjusted something as I believe my ambient was to bright, and actually the entire picture set was to bright. I should have got my meter out, but I didn't.......I do now. Lesson learned.
I think you have something to go on as far as technique, unfortunately you will have to try some experimenting before playing with actual customers. I would appreciate it once you have a technique down, come back and tell us......i would love to know exactly how you will be working with flash.