I participated in a loose airsoft LARP "Last Stand", based in the post apocalyptic Ontario, the NPC's posed small missions to search areas, get ambushed, kill other players, kill other NPCs... It was a blast.
A similar to heart game is being run in August, "Operation Hot Box" at the PRZ Picton facility, and will likely have a fair share of role playing.
The reason this role playing isn't as serious as "fantasy" LARP and more tongue and cheek is because the people involved aren't magical wood fairies, at least that's my take on it. We are, of course, dressing up to play soldier with toy guns.
Edit: MILSIM is not about LARP, it is about effective tactics, and a realistic approach to a military situation. I WILL roll my eyes if I see troops marching along...
Ár skal rísa, sá er annars vill
fé eða fjör hafa. Sjaldan liggjandi úlfur
lær um getur né sofandi maður sigur.