I posted something about this to a tax experts' website and was told you could probably also deduct the "convenience fee" if it's taxes you are paying on behalf of a business (employee's withholding/FICA/Medicare money).

I asked about how an individual could participate and was told not to try it, because to pay quarterly you'd have to file a false (or at least semi-false) W-4 saying you weren't subject to withholding and this was held to be an exceedingly bad idea.

I guess right before you file you could pay in some extra via CC and get a refund, but it doesn't seem incredibly lucrative.