I had always thought it was a servant, because whoever it was was available for dressing-up prior to the officers' arrivals in a group some time later. The head butler might not have done it, you're right. A stately, dignified butler would not have done at all for Lydia's joke, anyway: she needed a younger man to dress in drag. There was surely more than one male servant around Colonel Forster's home.

Would a soldier want to appear in a gown in front of his own C.O.? Surely, being the one with the lamp-shade on your head was not the best way to get a superior officer's attention if you wanted a promotion!