I guess in a sense, a hydraulic hybrid is "sort-of" an air hybrid considering what you are doing with an accumulator, only you can probably store twice as much energy or more in an empty air accumulator than in a half-filled hydraulic accumulator. an "air" car could be a closed system filled with nitrogen or c02 also, like they move the hydraulic fluid from high to low pressure tanks. Though you need a place/form for the low pressure n2 tank (could be part of the car structure I suppose ).
Another thought, if you need additional braking beyond what the motor is providing, you can transfer that braking heat to the low pressure accumulator somehow and assist the migration back to the high pressure side. Inboard calipers might help. Ah nevermind, not worth it, just stay off the brakes
