I believe that if you want to do something, you should go for it. Even if it doesn't seem like you have a shot based on precedent, the one thing I've learned so far in the military is that the only definite way to get a no answer is to not try. The worst thing they can say is no. Be prepared and have alternate plans but don't listen to people who tell you not to do it because it your chances are low or it hasn't been done before. I see people every year in the military get selected for fellowships that weren't listed as open fellowships to apply to but they submitted applications and the Navy made spots for them. Its rare, but it happens and if those people had just accepted that they shouldn't apply, they wouldn't have gotten to go do the fellowships they wanted.
Okay with that out of the way...
I do not know if the Navy will take PGY-1 civilian applicants, however, we are currently short on flight surgeons so that will probably work in your favor. I will say that if you are looking for air force vs Navy flight surgery, Navy is the more in depth training program and will get you more involved with the aviation community. Air Force flight surgery is a few week long course and doesn't involve any flying, Navy's is 6-7 months and you get stick time flying both the fixed wing T6 Texan II and the TH57 helicopter. The Navy residency in Aerospace Medicine is also open to flight surgeons who have finished their first 2 year billet. You do not need to have completed another residency first. I know plenty of people who went from intern year -> flight surgery tour -> residency in aerospace medicine in the Navy and loved it.