I've got my hands on a 14point7 UAFC https://www.14point7.com/products/ultra-air-fuel-controller
, which looks like it's going to do what I want.
Please note the edit in the first post, the ZC32s MAF is not a Karmen Vortex type, it is a voltage type. Voltage type MAFs send a voltage signal between 0 and 5 volts to indicate to the ECU the amount of airflow going into the engine, and the ECU adds the appropriate amount of fuel. The UAFC unit works by changing the voltage signal the ECU receives from the MAF sensor, which the ECU responds to by adjusting the fuel delivery, and thereby changing the mixture. I assume this only works nicely in open loop mode, but that's exactly what I want. It comes with a PC interface that allows for three different fuel trimming tables, 16 RPM rows and 16 voltage columns, so the adjustment can be pretty fine. The driver can switch between maps while driving, and it also does data logging with a couple of spare analog in channels.
At this point the unit is temporarily wired in, and I have loaded in a map with extra fuel added and have seen the lambda change both in the data logging and on the A/F guage, so at a technical level it works. We have a track day on ANZAC Day, I will do another update after that.