I seem to recall that some airline card I applied for gave me a temporary card number upon authorization which was only good for booking flights with that airline. But that would seem to be enough for you. So you may want to research whether the Chase UA card provides that benefit. (Sorry, I was unable to use that temporary account number and so I can't remember for sure which card for which airline it was, but I did apply for a UA card last year so it might have been that.)