I believe that the AF utilized the WO1 rank for a short period of time back during the early 70's. This was for Physician Assistants during their 2nd year of training. After training they receive their commission of 1st Lt. (Most of the PA's students accepted into the program were low ranking NCO's (E4-E5) in the 902X0 career field.)
I think you are correct but it only lasted a very short period. The AF remains oppossed to having Warrants. The feeling is they can get the same level of leadership and managment from Seniors and Chiefs for less money.