I received my card number online for a new Hilton Amex this past March. There was a note on the confirmation screen that it could only be used at Hiltons until I received the physical card. I did a test on an online purchase elsewhere and it came back declined. When I finally got the card it had a different security code than the one I was given online.

So I would say even if you could get the card number ahead of time you would prob only be able to use it on Delta purchases.