I agree Rob, for it to work using an injector PW the converter would have to be programmable with injector size, fuel pressure and be calibrated to output the correct signal so thats getting a bit complex.
If it can go inline with the fuel feed to the rail and be divided by four then that is far simpler.
Assuming the flow sensor can flow enough to not be a restriction at high load (same or bigger diameter as the fuel line) another problem is that on MFI it measures flow reaching one injector where as if we put it inline with the fuel rail feed it will measure total flow including whats being returned to the tank so will be over reading.