I really like this idea! As you write, it would be great for army builders, chess fans, and would help children learn to play chess. (Incidentally, I learned to play chess when I was about 8, so I think this could work.)
My only change would be to change the "trumpet bishop" to a "standard bearer bishop." I could easily envision an American Revolutionary War set, a Napoleonic War set, a medieval set, an American Civil War set, and an Indian War set.