Originally Posted by corporate-wage-slave

I don't know about twilight check-in and AA, but since you're in the Sofitel it won't be a big deal to give it a go (and report back here). For BA it's 16 hrs to 21 hrs. AA has its own check in desks in T5, clearly signed, they move they around from time to time but I think they are in Zone D, the end nearest the Sofitel. You will have to collect + drop the bags in LAX but the bags will be labelled to LAS, that's just a USA wide rule.

Originally Posted by guv1976

You won't be checking in with BA at all: you must check in with the operating carrier, AA in this case. AA should have special check-in desks for oneworld elites.

Your baggage will be checked through to LAS, but you must still claim them at LAX for Customs inspection, as LAX is your U.S. airport of entry. After Customs, there should be an AA re-check desk, where you will just drop your checked baggage.

You should check with AA, but I'd be surprised if you can check your bags the night before your departure.