IIRC, you can check in up to 24 hours before your flight including checking baggage, depending on the airline. That is, you can check in the day prior to your flight.

Strictly speaking, when you buy the AE ticket doesn't matter. What matters is when you put it in the turnstile to do the city check-in. The check-in counters are guarded by turnstiles that require an AE ticket (or will deduct the AE fare from an Octopus card). After checking in, you leave that area. You then have 24 hours to actually catch the AE train to the airport. If you use the same ticket to enter the train departure area within 24 hours, you won't be charged again. This isn't a "loophole" but a published feature of using the AE in combination with city check-in.