Ok, here is a thought, kind of plays on the drip idea, basically use a bike computer or whatever to keep track of metered units of gas. No real tight tolerances required except the solenoid valves need to work. Now just an electrical problem Again!
Will be near-real time depending on capacity of the resivoir and flow, I'm fine with a 10 second delay at full throttle myself if the valves will last 10 times longer as a result.
Assuming circuitry to make it all work (the controller), theory of operation:
start state: valve B is closed, valve A is open, fuel is flowing into the chamber and raising float C.
1. opto-coupler d1,d2 is interrupted by the float and the controller closes valve A and opens Valve B. The float begins to decend.
2. opto-coupler e1,e2 is interrupted by the float and the controller closes valve B and opens Valve A and sends a trigger to the counting device (i.e. bike computer).
3. goto 1
Still need to sort out venting the resivoir when the float is supposed to be moving...