I just realized there's a very simple and cheap solution to bias a narrowband sensor lean by 1.5% or so. This is not enough to meaningfully change best timing, EGT, or low load efficiency but it'll make a small dent in WOT operation and add ~1% peak power to most cars without the expense and complexity of fitting wideband controllers. On cars with a wideband, you only need to bias the rear sensor which provides correction to the wideband.
A voltage divider between a 1.2V reference and the original output, with a 7:1 resistance ratio will scale the voltage from 0.15 to 0.28, 0.35 to 0.456, 0.8 to 0.85, 1.0 to 1.03 etc. The reference should be close to 1.1 so that you don't get a fault code.
Since car alternator output is quite noisy, to get a reference voltage you can first find some cheap 3.3 or 5V step down converter then toss on a capacitor for smoothing and use a voltage divider. This reference voltage can be shared across all O2 sensors if you have multiple sensors.
The actual resistor values need some experimentation to play nice with the sensor. It may be the case that the voltage ratio has to be more like 10:1 so that the sensor still read lean without having to go way way lean.