Hi Adamey and thanks for your input,
The J&S I'm using was specifically made for cars modified with forced induction and includes two clamping voltage outputs for the O2 sensor. A fixed 0.5v and an adjustable 0-0.9v. Much experimentation with my particular installation has determined that the variable output should be set to 0.38v. These voltages turn on when the J&S senses boost. When there is no boost the J&S passes the O2 sensor signal unmodified to the ECU. My J&S is broken (it used to work) and when there is no boost it no longer passes the O2 Sensor signal. Under boost it still sends the clamping voltages. I want to use the fact that it still sends the clamping voltages as the trigger to some external switch device to toggle between O2 sensor and the 0.38v clamping signal.
Not to be contrary on this but I do know what I'm doing with the car side of things. Since the supercharger is aftermarket, the ECU has no clue what to do. A piggy back computer called a Powercard also senses boost and modifies the injector pulse to add more fuel to the increased charge air volume. The narrowband O2 sensor, as you said, can't measure this properly and just sees it as rich and it tells the ECU to dial back fuel. Now you have an ECU dialing back fuel and a Powercard adding fuel and they get into a fight the result of which is a lean condition until the engine gets to 4000 rpm and open loop (the ECU no longer uses the O2 sensor signal in open loop). Clamping the O2 signal at, in my case 0.38v, prevents the lean condition. In your example then Adamey, 0.38v would be interrogated by the ECU to equate to the 5.0ms injector pulse and it would stop making adjustments until boost disappears and the O2 signal is again passed unmodified to the ECU.