But when you put the information in it takes you to another page that shows the total price without subtracting the credit. On this page you can only confirm and place the order. I don't believe the promotion will work with this.
I did just buy online/download Microsoft Points from Amazon using an Amazon Gift Card I received as a "reward" from my credit card company. It did ask me to verify my credit card that is on file with them, but on the final payment screen I was able to enter the gift card code and it was used for payment.
Found this thread via a Google search because I was trying to do the same. Yes, you can use credit from previous pre-orders to purchase XBL online codes. I just did it to buy the 2400 point online code. I paid $20 (plus tax) instead of $30 (plus tax).