Actually, AA has a policy about this. I know because I've done overnights in SFO twice now to get the early HNL flight. One time I got there midday and had to reclaim my bag. The other I got there at night and didn't. Basically, they say that there is a specific time that if you stay longer than that then you must collect your bags. I believe the cutoff might be 12 hours, but I can't say for sure. I just know that the flight with the shorter layover they would not shortcheck but the one with the longer layover they were required to. Of course YMMV, but AA does have an official policy in place on this. The computer will not let them check it through if your time on the ground is more than x hours.