One approach, slightly different, would be to buy a flow meter used with other types of gasses, such as argon, and then re-calibrate it for Hydrogen and Oxygen.
Another method, for really high gas production, (resonant action type awesomeness), would be to have a water level indicator integrated into the cell, but isolated for the disturbance of the rising gases, and then have a tall thin tube on the cell so you could actually watch the water level drop as gas is produced. Then factor it 1:1800 or what ever it is exactly, to get your measurements.
When it comes right down to it in a real system, it doesn't matter, all that matters is that you "have enough" and then can adjust the production accurate to satisfy demand.
