Another possibility is to have the flow rate set manually very high, and have a solenoid valve before that that listens to the printing output for the 'laser on' signal and shuts off the flow when the laser is on for more than a few hundredths of a second (to keep it from fluttering while the laser is rapidly turning on and off to make a multi-line image.)

Making sure to have a simple "laser is on" signal output from the Peachy would make this (and other hacks/accessories) even easier, since other devices wouldn't have to listen to the signal from the computer that way.