I know what you mean...I have found this system (thourgh a R/C Tank website), its a little more than I was willing to spend but it should provide about 50psi or 3atm...maybe less.

http://www.palmer-pursuit.com/online-catalog/rock.htm

I only need a little air, so I am currently planning to use channel 5 (landing gear channel) to control a 5v air pump. I am waiting for the regulator to be delivered so at the moment its all just an idea!

By the way, my model is a SeeHund...I think I may have found a way to disengage the torpedos...with magnets!!! Another funny little idea, if it works I'll be impressed...

Problem is I need a counter rotating gearbox with concentric shafts...any ideas?????