I actually met with an AF recruiter about a month ago. I've been accepted to med school, and I am very interested in ENT. While speaking with him, I told him I was interested in Peds, and he said there was great opportunity for peds in the AF. I then referred to the fact that my father is an ENT doc, and said that I didn't think I was interested in that (though I had not ruled it out at the time). He then informed me that its probably for the best that I was not interested in ENT, because there's no real need for it in the AF.
So, I guess if I was interested in ENT and already in HPSP (which I've decided I'm not going to do) I would just serve as a flight surgeon, and then apply for ENT residencies after I got out.
Best of Luck!