there is a hole in the air filter indeed. the spray is injected right through this hole.
For the tank you mount a check valve in the pressure supply line. Once the container is pressurised, it stays pressurised. 20l or about 5 gallons of compressed are generated by your turbocharger within some 200 Milliseconds for a 300HP air flow.
After that you only need to replenish the air replacing the fluid.
One example found here
http://www.sprayernozzle.com.tw/down.../CTGAZ18BR.pdf
sprays 500ml/min (30l/h) using 60l of air per minute or 1l per second at 0.7 bar. At 300Hp your car needs 250l of air per second! The loss is insignificant. The same is valid to fill the container.
Hydraulic nozzles are spray nozzles driven by high fluid pressure. It is what most of us are using.
Working with not too big pipes (for 300HP, 50mm is still OK), not too bog of a IC and a blow off valve, lag due to a intercooler should be minimal on full chat. I see the simplicity of your approach and plumbing a IC is a lot of work. I also tried just WM injection, but had worse results compared to both CC and water injection.