Oh dear.

I guess it would be helpful is someone who is scheduled to fly out of DFW from Terminal D in the morning -- preferably somebody who did NOT need to check luggage and was just headed to security -- took a look to see if there is actually a Frontier ticket counter in D. Frontier sent me this email before my flight, but of course I don't entirely trust them. I suspect that if they do have a ticket counter, it's in the same place where the international airlines have counters.

BTW, if you have lounge access, it's worth noting that the AMEX Centurion lounge is in Terminal D, so that's not a bad place for your Frontier flight to leave from. That lounge opens at 5:30 am, with hot food at 6:30 am. I understand that there is a new Priority Pass lounge (Plaza Premium) in E that also has hot food, but it is reportedly small and quite crowded.