The system should work the same. The O2 sensor data will determine the a/f mixture and the ecm will compensate accordingly by adjusting the integrator (injector pulse width). For a larger displacement, the O2 sensor data will show a lean condition from baseline and the ecm will increase the integrator, lengthening the injector pulse until it makes the O2 sensor happy.
The system is VERY forgiving as it has to be able to adapt to extreme conditions. Personally, I think it would work just fine without any "tricking"
The basic principle of FI engines is that the driver controls air flow (throttle blades), then the computer looks at air temperature and pressure to derive density, applies that value against known displacement to determine combustible air in the manifold, and finally injects the appropriate amount of fuel. As I understand it, the O2 just modifies the curve and doesn't override the calculations. Of course when the displacement goes up, the air volume changes, so the calculations are thrown out of whack. In the jeep world the common fix for this problem (strokers are pretty common) is to tweak the MAP sensor to trick the computer's air density calculations. Every thread I've read about the subject here talked about using an adjustable FPR to boost the fuel quantity for each pulse. I suppose either would work, some other things might too.
How's it gonna run the engine before closed loop when it starts using the o2's feedback? A smaller FI system on a bigger engine will run lean until the o2 sensors 'wake up' and tell the computer something is wrong. You might try bigger injectors to compensate and then rely on the system to dial the fuel back after closed loop. Just an idea. It wont be 100% plug and play....
I assumed the learned long term fuel trim that is stored would take care of warm up. While short term only takes care of closed loop operation, like you mention, long term takes effect in both open and closed loop. Of course, then it would run horribly whenever the battery is disconnented...
But yes, ehalls comment makes sense. If it wouldn't work, I'd head over to the fiero guys and have someone build you a custom PROM rather than messing around modifying sensors.
Agree with carnut. The 4.5 swap is much less finicky. Not quite plug and play, but close. Look up some old posts from Illumnati from about 3-4 years ago (or PM him if he's still around). He did this swap a number of times, and had it down to a science.