Hi Paul,
Roy and I have toyed with such an idea, to the point of identifying suitable cylinder rams. They can be sourced from as little as 10mm bore of varying stroke. We considered using a bottle of compressed gas for mobile testing purposes and a diving bottle for bench testing. (both suitably regulated to say about 8 bar) The small disposable mig bottles you can get can provide 100psi+ and could possibly be the basis of a resevoir topped by a small electric compressor.
When we went to SPA a couple of years ago, we came across a caterham or similar that was indeed fitted with a pneumatic shifter (USA sourced) that was contolled by a small box of electronics via two push buttons on the steering wheel. The system was all the more useful for the driver to whom we spoke with, as he had unfortunately lost an arm in a previous accident (motorcycle?). EauRouge in a 40 was great but with only one hand on about a 10" steering wheel - very brave!!
Good luck Paul and keep us posted.
(ps - keypad is being posted tomorrow - sorry for delay)