[Phantom]

Instead of sucking air in or blocking the intake as that would introduce a restriction making the other working cylinders work harder you could try one of these options.

1. Reroute/route exhaust to the deactivated cylinders controlled by a butterfly valve, the exhaust would be ran from just a little after the O2 sensor before the cat. This would allow the O2 sensor to be happy as no new O2 in being ingested. The issue that might happen is leakage of exhaust gas when running on all 8 that could be reduced if the new line is added as a T with an additional valve to block the exhaust.

2. Is the opposite idea of the first. Let the engine take in all the fresh air it wants but modify the exhaust on the deactivated cylinders to vent to air. Once again use a butterfly valve so you can change the exit of the exhaust this way when deactivated the valve will move blocking off the original exhaust and direct the air out of a short pipe. This option should take care of the O2 sensor problem also as it will not see the unburnt air since it never makes it to the sensor. Doing this you might need to have the firing order even so the O2 does not have a large gap with out a reading. The other issue that there might be is fresh air being pulled into the exhaust or worse exhaust being vented to air. Air being pulled in would be easy to fix with a flapper at the end of the pipe but I doubt that will be an issue so you could add a second butterfly valve to the exit of the pipe to help ensure that is is sealed.