I don't actually see why it would be wrong to give an imaginary credit card number, if the whole "transaction" is imaginary. It's not fraud, in my view, because YOU'RE NOT BUYING ANYTHING. When and if you plan to buy something, you can update your credit card number - I'm sure they'll be happy to receive it.
Another option, if you're determined, is to get a new credit card, submit the number, then cancel the card.
Just my opinion.