Tim, your linear valve is absolutely incredible. I just don't know what to say, its just so good.
If this thread is still open for new ideas, I have an interesting one: a kink valve. Basically attach the pvc to a stiff garden hose and go back into pvc again. The two pvc sections connect together like a rail system, where the hose is kinked in the off position and you move your grip forward to unkink it and fire the gun. Or alternatively you set it up to where pulling back a pvc section unkinks the hose. We can finally take advantage of crappy garden hose!