Jon;
The Aqua Sub, designed by Kent Markham I think, used the valve from an automobile lift... forward to lift (blow), back to lower (vent). It's what you describe, with one lever for the two valves, but my concern would be the flow rates because it's built for hydraulic fluid, not air.
Stan Freihofer
George's ballast/vent plan is simple, but requires many
valves. I'd love to see a manifold system with a double throw valve so
that you push ONE valve forward and it vents, push that same valve
rearward and it blows, no air flow in the center position.