Make the reservation and select hold. Then call the AA.com, give them the reservation number and tell them you are paying with a voucher. They will confirm the reservation and give you instructions on where to mail the voucher.

You meet any advance purchase requirements when you call but you won't get ticketed until they get the voucher. Obviously, this only works if you have a couple of weeks between the trip and the reservation. I've done it before and it worked fine.