We are told that Col. Forster knew about it ahead of time:

"Not a soul knew of it, but *Colonel* and Mrs. Forster, and Kitty and me, except my aunt, for we were forced to borrow one of her gowns; and you cannot imagine how well he looked! When Denny, and Wickham, and Pratt, and two or three more of the men came in, they did not know him in the least."

As someone pointed out at the P&P board, it makes you wonder if Col. Forster was quite such a "sensible man" as Mr. Bennet thought! I still think it was a young officer, at least partly because it would have been a bigger joke for his fellow officers not to recognize *him* than not to recognize a servant whom IMO they would have regarded as part of the furniture, anyway.