Is there any of their works which involve a sub-lietenant being found in a wimple???? I am a fan but don't remember this storyline- been suggested as quiz answer, but not convinced!!
None of their operettas involves exactly the scenario which you've described.
However, some of them do have far-fetched plots whereby one of the main characters has had something unusual happen in infancy, i.e.:
"Ralph" + "Capt Corcoran" in HMS Pinafore
"Frederic" in Pirates of Penzance
"Luiz" in The Gondoliers
Can't think of any others offhand but there may be one or two I've missed!