The ball shifter will not “pop out of gear” right away, but over time it tends to move and will eventually pop out of gear. This could be a few seconds or a few minutes, but we’ve had this happen plenty of times when we forgot to pressurize the system during practice sessions.
Adding a spring to hold it in one direction if there is no pressure would be a great idea. But if the spring was trying to hold the shifter rod in the “pulled out” direction and the pressure was lost when the rod was in the “pushed in” direction. The spring would actually cause the shifter to come out of gear. If the spring was strong enough to push it into the opposite gear, then you are good. But it would need to be more than a light duty spring to do that.
You could add an accumulator that was directly connected to one port of the shifter cylinder and pressurize that accumulator to 30 psi. The other port would be connected to a normal pneumatic channel. When activated, the 60 PSI working pressure would push the cylinder piston with 30 PSID of pressure (60 on one side and 30 on the other). When you de-activate that channel, the cylinder piston would push the other direction with 30 PSID of pressure (0 on one side and 30 on the other). The 30 psi accumulator would “bias” the cylinder in one direction.
The accumulator would not need to be as big as a clippard tank. It could be a much smaller “tank” as long as the volume was large enough that the pressure wouldn’t change much when the shifter cylinder changed positions.
If you have a regulator to step the system pressure down to 30 PSI into this accumulator tank and then a check valve to prevent the accumulator from leaking back into the main system if you lost main system pressure, you would be able to pull this off. Edit - as long as the vent valve is downstream of this second regulator, you can still dump the entire system pressure.
Edit: brain fart - the vent valve can still be located in the system to be able to dump the whole system. Sorry for all the convoluted attempts to design a legal system in my earlier post. They were unnecessary and I have removed them for my own sanity and to save you from having to read the ravings of a madman.