Since I know most military focused BBs like to keep a rank system, why not make it a little more realistic and award based.
Rank is recieved/awarded/earned by points (number of posts), performance (informative/worthwhile content) and TIG (date you joined the board). Why not up the posts to 100 per each rank. Start as cadet, after 50 posts, PVT, 50 more posts, PFC .. ect. As the ranks climb, the post requirements could become longer .. say one has to have 200 posts to make LT. (This idea, of course, assuming it's possible to mix up the post requirements) You could also add a voting system, those that contribute regularly, with informative and insightful posts could be voted into a higher rank. A bit like your "donate milbucks" idea. So, one could be a Sgt, but those within the web community feel they are a great asset to the community .. they are awarded for contributing .. and thus obtain whatever rank that correlates with how many votes required. IE. Five or ten votes = LT .. or whatever.
This way, you keep in the tradition of what a rank systems if for, giving the more frequent posters (those that contribute the most to the web board) the higher ranks .. and those that don't, the lower.
Perhaps for those that are active, or former military - after a certain number of posts they could purchase their specific rank (after an introduction of themselves with an overview of their service) with their milbucks and keep that specific rank as more of an avatar for themselves, if they so choose.
Just an idea, may be too complicated.