[Phantom]

I think you would have three methods for doing it and having the computer play nice.

1. Get a O2 spoofer these are usually are used to make the car think the CAT is still there or used when you do not want to spend the money on the post cat O2 sensor.
Wire up a switch so you can turn on the fake O2 when you turn off an injector. With some work you could possibly use one switch to do both.

2. Depending on how the exhaust is routed move the O2 to read only one bank so it does not see the deactivated injector. This would take more work and would not be able to adjust for unbalanced fueling on half the engine.

3. Depending on how the PCM/ECU operates it might be possible to change the tables it uses by adding a switch. That is only easily possible if it had something like "sport" mode available that already does that. IIRC some GM engines like the 3800 you could move the programming to Speed Density mode by disconnecting the MAF and I believe the system ignored the O2 for fueling and only went on MAP and RPM. The down sides not all PCMs had decent fueling tables in SD mode and you could not get back to standard fueling without turning the car off.