I would think that the ability to pay is tied to the room account, you should be able to charge until midnight of your last day (or even later if a park is open past midnight?).
I haven't heard that the ability to pay is separate from the room account - yet.
Payment is tied to room reservations and as of right now will not be allowed for Annual Passholders without an active room reservation.
I'm not sure about using as a payment feature. There's a way of putting a credit card on your MDE profile. *Presumably* that can be accessed via the Band, but I haven't read of anyone trying it. If it did work, I'd presume that every charge would go through individually rather than build up like a room charge account.
After looking into that section of the MDE website more, it seems that storing your card in your profile is just to use it for resort and dining reservations as well as buying park tickets. So ignore my prior comments that I've quoted. (In the future it'd be nice if Disney could allow you to link the card to your profile and just run the charges through individually if you're not staying on-site, but I'll bet there's a lot of fraud liability involved in not having the card in-person at the register).
-Rob
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