I've been seriously thinking the motor-controlled syringe concept and how I could improve the concept to an acceptable point and then make a cheap prototype. I have the experience and ability to make all the required control electronics and software (to have a feedback from the measured vAg, for example), and the mechanics is not impossible either if I can keep it simple. However, this all will take time as I have many other projects going on. But, if I ever get a working prototype under construction, I'll be in contact with you, PE!
Basically I would need three solutions - AgNo3 solution, Br solution and I solution, correct? Allocating three syringes for every solution would make total of 9 syringes. They would be controlled by linear stepper motors that cost about 15 USD each. IIRC the ones I've been planning to buy can be controlled by 0.024 mm steps. With small syringes (down to 1 ml or even smaller, if needed) you could get very slow and steady stream. OTOH, 50 ml is the biggest easily-available cheap syringe. I have 25 of them and they cost about 1-2 USD each. With three syringes per solution, for example, 5 ml, 20 ml, 50 ml, you could get total 75 ml of each solution with very flexible flow rate range. Syringes would be connected to very thin tubing, e.g. 0.6 mm, 1.0 mm, 1.5 mm respectively. You would lose only a ml or so of the solutions in this short tubing. If there is problem with solutions dripping out instead of spraying, very small nozzles could be made at the end of the tubes.
This won't scale up much, but I bet it could be made to work with small volumes (0,5 liter or less of emulsion?) quite well, but it would take time. And I don't have the necessary know-how in the modern emulsion making, which is a problem. I would be very thankful if you, PE, could post some examples of the exact pumping ramps/curves, so I could get a better idea what's needed.