You could have multiple one way solenoids, each with its own regulator. Any weirdness could be fixed in code.
The way it would work is that you use multiple single function solenoids (or just a standard two way with one output blocked) to feed air into the solenoid and one solenoid as an exhaust. This setup would have to be repeated per function (in, out, etc). It would be ugly, but it would work.
If you find a way to reliable change air pressure I would like to know
A bit of googleing (yes... google as a verb) found these (
http://www.smcetech.com/CC_host/page...6669613456b3f5)
They are electronic regulators, don't know if they are good for a robotics aplication, but it would be interesting to try.