I recently bought an Oxford World Classics copy of P&P, with an introduction (copyright 1990) by Isobel Armstrong, and while making another argument, IA mentions "Lydia, who jokes about the excitement of dressing *an officer* in drag" (p.xxv, my emphasis - L.). I don't know who Isobel Armstrong may be, but here's another person who thinks Chamberlayne is an officer, not a servant. :-)