Figured this out today and thought someone else might want to use it as well. Setting a BATTx_CURR_PIN to an aux pin (50-55) allows you to use a hall effect fuel flow rate impeller. Some more details are in this commit by @tridge, but everything is working fine. The only thing that isn’t too great is on a CubeBlack 3.9.8 only has 2 battery monitor instances enabled. So this comes at the cost of loosing current sensing from one of the power modules.
@TunaLobster master includes support for upto 10 battery monitors at once, so that will be available in Plane 3.10. (Using 10 battery monitors can of course be a problem to find that many analog input pins, but if you are using the pulse inputs, smart batteries, uavcan monitors then you can easily start to fill these up).