So I was watching the credits to Infinity (looking for the cast), and, at the end of the credits is all the legal information stating that the aircraft in the game are used under license from their various manufacturers. Anyway, in the Boeing/McDonnell Douglas section the F-15J is listed along with the rest of the Boeing aircraft featured in the game. You can also see it at the bottom of the main page on the Infinity website: http://www.acecombat-infinity.com/.
So could this be a hint at future DLC?
I know there are allied JASDF F-15s in the Tokyo missions, but they simply appear to be F-15Cs and it doesn't really make sense to add the F-15J just for that.