There is a unit made by Jacobs Electronics which is very highly regarded in America. The unit activates N2O at a predefined RPM level, protects against weak fuel mixtures by shutting of N2O if the fuel pressure drops too low, and also allows you to map your own timing curve!
I have never heard of an application that works in proportion to RPM. The nearest thing I know of is a 2 or 3 stage nitrous kit. Additional stages (i.e. 2 or 3) are usually triggered by the user by a button on the steering wheel etc.
Hope this helps
Laurence.