coolash: this is a great thread. thanks for sharing. your idea is super cool. How's your 2nd iteration coming along?
Here's my bit to give you some more ideas:
This bottle or the Resqteq hand sprayer should work flawlessly as a foam spray without needing any significant mods if one can source a 110 degree foam nozzle head & it's housing locally. Shouldn't cost more than INR100. It's mostly used by carpet & upholstery cleaners and in the hotel/hospitality industry.
I know a supplier in UK who will ship this set for approx INR900 to India:
Of this set, the orange nozzle is the one of our interest. That's the 110 degree wet foam nozzle.