I thought I'd update this thread with the information I've learned.
I checked APC's site again for the BR1500G UPS to see if anything had changed. The PDF document on the model added info on the replacement battery cartridge, but they still don't sell it. I decided to query them on the situation, and their reply from "Devendra" in somewhat imperfect English was:
I am afraid to say that the replacement battery for this unit is not released to purchase, because this is brand new unit and this unit will carry 3 years of warranty from the data of purchase(this warranty will include even the battery). So APC will take care of this unit and battery for 3 years. This battery will be released after one year.
The main reason for not releasing this battery is, customer cannot stores any battery for more than 6 months, because storing battery for long time will loose the battery life and even may lead to damage in battery body.
I also learned that the model is available at Wal Mart, so even though it will be an extra ~$24, I'll likely get it there, and soon, because it means inexpensive returns and likely less trouble. Though, it will likely mean waits because I think my nearest store won't carry it unless I have it sent there. But I've been waiting far too long and have already had enough trouble because I'm nearly at the maximum 15 amps my outlet can provide and the window A/C I have in the room sometimes crashes the circuit breaker, which is... unpleasant, to say the least.