This is just me thinking aloud, but how much does that control box do that a smartphone running an appropriate app & plugged into the the probe couldn't? If this was feasible, I could see it being possible to integrate your metering with relevant documentation - ie how much + or - development etc - & even automate that process to a certain extent, perhaps incorporating tested film curves in the app to enable you to tailor exposure to specific films in a convenient manner. The ultimate stage would be to integrate this with an electronic in-lens shutter, with the smartphone as the controller, but that's perhaps getting away from your design goals...