I may have a novel idea, or an unrealistic idea; I don't know. Have a venturi suck the water mixture into the air stream. Pressurize the water's holding tank with boost. Have a simple valve between the tank and the venturi. Could you use an idle air control valve for a simple/cheap solenoid valve? Oh and by venturi, I mean old carb. Want more water, change a jet size.
I have tried multiple different water/alcohol systems(all DIY) but I have settled on a fairly simple and cheap system. I use a Clippard Mouse normally closed solenoid (which I have found to be light years better than a check valve). Your ccm needs are pretty low so I wouldnt worry too much about the pump as all of the ag sprayer pumps are grossly over sized, pretty reliable, and cheap. Whatever cheap pump you get that can do about 80 psi will work just fine. For the controls I prefer referencing injector pulse width as it follows load far better than boost pressure. A simple trim pot so that you can set the reference voltage you want the solenoid activated at will work just fine. You can get more complicated from there making multi stage multi solenoid systems, but I couldnt prove any benefit from the added complexity. I also couldnt prove any benefit from fast solenoid valves to control flow.
The one thing that I can say didnt help was varying pump output with a pwm controller. At low pressure the nozzles didnt flow right no matter which nozzle I tried.
The only system that I could never get to work right was a pressured tank system that was fed by a boost line from the turbo. I couldnt get it to maintain a consistent pressure in the tank and lines. My observations were that I was getting varying boost pressure from the turbo and boost control solenoid in conjunction with the boost recirculation and all of the sucky check valves( I never have been able to find consistently high quality check valves for cheap). I also had problems with leaking tanks. So in the end I went back to a pump system which has proven to very reliable.
donalson wrote: to Zomby Woof... what did you pull the SS injector/pump from?
The ones I used were not SS, but I know there are some available from late model applications. When I was planning my most recent setup, one of my buddies offered me some SS injectors, and told me they were becoming much more common in the later stuff. I'm not really up to date with who's using what for injectors these days.
If I get time later, I'll post up some good links for putting a budget system together.
In reply to Zomby Woof:
I would like to see something put together with injectors too. I imagine you could put together a much cleaner and simpler system together that way. I have often wondered if distilled water with alcohol and a water soluble oil would work with a fuel pump to supply the system.
In reply to PseudoSport:
Ah, those can't be modified. I tried some...
In reply to alfadriver:
That little trick is very popular on subarus
ALFA... what sort of injectors CAN be modified?
Im not ALFA 
...but... Google some Denso injectors for Subarus and you can see how the end of the injector is covered by a cap. The injector pictured earlier is recessed and does not have a cap on the end that can be cut off.
Interesting - I've had this same subject in mind recently.
My thoughts were:
-Nozzle fitting with a fine but wide spray mist (like an adjustable garden sprayer nozzle on wide spray mode) -Continuous pump, hooked to a manual switch -Large windshield wash container -Vacuum hose
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