This is still a WIP, but I believe it fits generally with what you're looking for (Sherlock and John going undercover as a BDSM couple who really have very little idea of what they're doing; Sherlock as the sub, John as the dom). And, God, it's amazing.
Re: Johnlock, going into BDSM undercover, get into it for real
http://archiveofourown.org/works/868878/chapters/1667981