I like this idea very much as a class of an advy. (perhaps the character types should now be "noble" and "commoner", with "adventurer" and "captain" being commoner classes)
I can see a lot of reasons why an adventurer might want to go join the army during wartime. i don't see the class having enough reason to exist when there isn't a war.
I wasn't clear if you were proposing this as a separate "person" that can go find a noble to attach himself to on his own, or one that a player can create to go along with his first, noble character, but would never move around on its own. if the first, how would the captain even be able to play if he was attached to a semi-active noble who never made it to battle? it seems too dependent on the activity level of another player.