[BushTA]

I've never done this, but have always wanted to try it out (like in the off season). You use a 3-position, Center closed, Solenoid Valve such as an SMC SY3340, with Flow Control elbow fittings to limit how fast the piston moves, and power the two valve coils from the outputs of a Spike (tie both coils negative leads to ground)[this also prevents the dreaded powering of both coils at the same time]. When the joystick is centered (null) have the Spike Off (no current to either coil) this will close both the extend and retract valve output ports, holding the piston still (mostly). When the joystick is pushed forward, power the solenoid that extends the piston. When the joystick is pulled backward power the solenoid that retracts the piston. I recommend that you use meter-out flow control elbows and put them on the valve manifold ouput ports. How well this holds a mid stroke position will depend on the load and probably whether the Driver was extending or retracting the piston prior to trying to stop it. It'll work better in the direction that is pressurizing the cylinder against the applied load.

Good Luck,