There is some wilderness medicine research coming out of Madigan, and I have heard of a few people working on the fellowship on their own time. This is not something for which the military will pay, however. You can get credit towards the fellowship by getting the military to put you through some of the military medicine short courses (Tropical, Mountain, etc). Talk to Perrotfish, as he's going through the process. I do not believe it will do anything for your career advancement, it would just be something you'd do for fun in your own.
There is no utility in EMS fellowship in the military.
It's worth pointing out that I think the part time / own time "fellowship" that psychbender is talking about is likely the FAWM from Wilderness Medicine Society, which you get from satisfying CME and life experience hours. There are also actually legit 1-2 year post-residency fellowships (like the one out of Stanford). The former doesn't hold much water but the later does for a very select niche. And I can't imagine the military caring much.
Madigan does have a formal Wilderness Medicine Fellowship. It seems it opens up career tracks in the military towards teaching others about austere care and developing new methodologies and certification programs for CLS courses.