For future reference incase anyone is looking for the same information, it looks like the CIS computer cuts power to the O2 heater circuit under load. I drove around with the multimeter hooked up to the wires and was able to characterize it's behavior.

On a cold start, the O2 heater circuit does not get power for about 15 seconds. I don't have an accurate time for this because I wasn't expecting this to happen and didn't time it. I'm guessing this action is to purge the condensation so the O2 sensor doesn't get destroyed from starting the engine.

Under heavy load conditions, even before reaching operating temperatures, the O2 heater circuit is cut. It's not RPM based since sitting idle and revving the engine slowly keeps the heater circuit alive. If I floor the gas pedal in 2nd gear idling, the heater circuit gets cut even at 2000 RPM.

So to answer my own question, I'll need to find a new power source for the wideband O2 sensor. I'll either modify a MAS relay if I can easily figure out what to do in there, or just run new wires from the cigarette lighter.