Is it possible to put QEV's on older LCDs and LEDs? Possibly replace air assists and be mounted on the sight rail? I'm not sure how much performance gain if any would occur, but it's interesting to think about.
Not really. QEV's are generally mounted to the inputs on the QEV's to allow the solenoid to vent the air quicker as it switches. To see any kind of improvement, the solenoid pathways will need to be the main thing constricting the flow of air.
Is it possible? Sure. Do I think it'll really make a difference? Not really.
If you want to set off and go develop some grand new thing, you don't need millions of dollars of capitalization. You need enough pizza and Diet Coke to stick in your refrigerator, a cheap PC to work on, and the dedication to go through with it. We slept on floors. We waded across rivers. -John Carmack
Since the solenoid is manifold-mounted directly about the ram, where would you even put the QEV? I guess if you made a custom manifold, but that would probably make the solenoid too tall and it would stick up out of the housing. Technically the LCD solenoid is 5-way since, IIRC, it vents in both directions, so a QEV would be redundant anyway.