I have not seen the two systems combined, but it may be out there. I do have experience with the SNOW Performance water/methanol setup and can say it's well worth the money and the tech support is excellent.
My thinking is that the system is used to reduce detonation so if it had a knock sensor that would tell the system there was detonation then it could inject more water / alcohol until the detonation got down to an acceptable level. It seems that it would be a better way to control then trying to set a boost level where the water / alcohol starts injecting. It would also automatically adjust if you got a bad tank of fuel.
I use water/meth injection on a supercharged 3.8l which turns on at a certain psi. But just thinking about it, if the mixture comes on in junction with a knock sensor, won't the damage already be done? If the sensor senses knock, and the mixtures comes on, the engine is already knocking, which isn't good. JMO