Air over hydraulic is a proven system. Essentially it is an air brake system for the control part (the pedal) and a hydraulic brake system for the actuation part, the wheel cylinders.
To make it work there is an air chamber directly mounted to a hydraulic master cylinder.
Your combination system should then have three air chambers, one for reach rear wheel, and one on the master cylinder for the front hydraulic brakes.
I have only talked to a very few people who thought air over hydraulic brakes were a good idea, and actually liked them.
The impression I get from everyone else is that hydraulic brakes are OK, but nothing special, and air brakes are great, but together they are not worth having. You have the complication of an air brake system, which by it's self is OK,over the frustration of a hydraulic system, it's a way to have good brakes when they work, and a good way to spend money and waste time when they don't.
I briefly considered air over hydraulic brakes for my project, till I did some research and found out the above. I have hydroboosted hydraulic disc brakes now, and they are great. But given the chance to switch to the new air disc brakes I will, but looks like 5 years or so before "small" air disc calipers are readily available in the US.
Grigg
