You couldn't do it in the same way as in STW or MTW, but it can be done differently. Imagine the Prologue, but pretend on the campaign map that your banners are moved for you. Basically, it'd all be done by a script that would control the campaign map and let you fight on the battle map. I know The_Mark's been looking into this sort of thing, Myrddraal would be a good person to talk to as well.