Traxxas has a kit like that for their GTR shocks, variable damping kit. I have the kit on all 3 of my revos. I'm not sure of the piston diameter, but it does work really well. On compression, 2 of the 4 holes get covered, on extension, the reed valve lets oil flow through all 4 holes. Really helps the revo land bigger jumps without hitting the ground so hard.
Never thought about it... but that would work on my jato too! How did I go this long and not realize that?!?!