The "sneaker line" of the sort you're looking for is (or was some time back) a more or less standard thing in barbershop singing. Most of the trick lines were "naughty" at most, rather than outright obscene and nasty.

It's been many years since I was regularly in the company of active barbershoppers, but I can recall a late fifties SPEBSQSA convention in the mid fifties at which one of the favored quartets did a "zipped up" version of School Days. The first run through was straight, and in great harmoney, followed by a second pass with a number of "zingers" slipped in.

Sorry but about the only thing I remember for sure was the line:

"She wrote in my book, I love you true, when we were a couple of kids."

became:

"She wrote in my book, ...poignant pause ... I know-a-dirty-joke,
when ...etc.

Rather tame, I'm afraid, but I suspect the practice survives if you can find someone active in that tradition.

Down by the Old Mill Stream is another often done, in the "patter style" with interjected counter-lines.

Down by the old mill stream, where I first met you ...

becomes, usually with the high tenor doing the italicized bits:

Down by the old not the new but the old mill stream,
Where I first not second but first met you ....

The interjected patter lines were frequently "warped" into suggestive form, but I'm sorry, I can't remember a version to pass on at the moment.

Certainly the most "risque" zing was when another (or maybe the same first mentioned) quartet replaced the traditional ending tag "Shave and a haircut, four-bits...etc. with:

Sung by the whorehouse ... quartet
Have ya got a hard on ... not yet
Are ya gonna get one ... you bet
slowly and in full harmony It's sloooowly riiiisiiiing.

(note: high tenor modulates on last syllable)

John