There are some pieces of misinformation which are driving some aspects of this discussion.
First, Apple has said in separate interviews that the unknown AirTag alert doesn't trigger until you actually get home with one, or reach some other "significant location" based on your previous location data. I've interpreted this to mean other places that you frequent, such as work, a favorite restaurant, etc.
Second, the cargo hold and the passenger cabin are only separated by the floor, which is thin and radio transparent. Passenger aircraft are basically pressure vessels with wings. All the structural strength is attached along the inner wall. So yeah, if we're talking about an A380 you could be out of range just due to physical distance but not much else. But an average 737, I really doubt it.
As for the rest, refer to the conversation regarding Tile. AirTags are really no different, so I fail to see why they would be disallowed.