Is Lion's new "Date Added" field supposed to work in the Trash?
The answer was basically in my original question. Spotlight is indeed the sole arbiter of the Date Added column. And the Trash, not being a real folder, is not Spotlight indexed.

If you move a file with a Date Added into the Trash, it keeps the Date Added from its original folder. If you then put it back, its Date Added (to the original folder) is updated to the present.

It would probably take a significant engineering effort to make this work for the Trash, so I'm not expecting to see this fixed before Mountain Lion, if ever.