What if you did something like this...
Custom 50x100 Cyclinder to Buffertank link.jpg
With the pneumatic cylinder we have you can take the end off. If you took that off then replaced it with a custom valve that had a diameter that was the same at the cylinder bore, I am guessing that this would technically be the best possible set up.
You would weld a custom mount to the side of the tank then have a solenoid inside to activate the valve itself.