Yeah not a whole lot on the military forums, check out Jawbreaker's post, he answers a lot of Army oral surgery questions. I'll help you out as much as I can. You need to do the same things as you would for a civilian residency (high rank, great CBSE, extracurriculars, etc.). With that said I would say 90% of your app is rank, CBSE, letters, interview, the extracurriculars don't matter nearly as much.
You can and should extern at military programs. I would do maybe 1 week at a good civilian program to get your feet wet then do a couple at military programs. You will need to do 3 weeks for the Army to cover your costs.
PM if you want a write up of the different programs or have any other questions.