I don't think that EFI and mixtures are the only thing that causes fuel dilution or oil degradation and, thus, cannot completely eliminate it. When idling, there's almost no load on the engine (AC excluded). This can lead to less ring seal because of "low" cylinder pressure. This may mean that there's a lot more blow-by into the crankcase when idling. This can carry some fuel and all of the other acid-causing combustion nasties into the oil in higher amounts than when ring seal is better. I suspect that during this period more "crud" would also be getting fed back via the CCV/PCV system than in normal operating conditions like highway cruising. I would also expect idling to promote carbon buildup since it is the polar opposite of an "Italian tuneup" in the hilly twisties.
Idling is probably always a bad idea, modern EFI or not. Modern EFI can only watch for unburned fuel in the exhaust gases, not the crankcase, and doesn't account for all of the other negatives. In some situations it's unavoidable, and even cars who do it a lot can make it to high mileage. It may come down to your OCIs, your oil's ability to combat contamination, and your general maintenance regimen (fuel quality, fuel system cleaners).
Another factor of potential concern is whether your particular engine model was designed to provide sufficient lubrication pressure/volume for extended periods of idling. I would not make the assumption that this is in the design criteria of every engine manufacturer like it would be when designing vehicles used for taxi and emergency response uses. In the situation describes by the OP there will be a LOT of heat soak and very little airflow. Despite low engine load temperatures could skyrocket in parts of the oil (and water, depending) system.