I googled some of your suggestions in case it was US slang or something I wasn't familiar with but looking at the results I don't think it is.
Venus on the half shell - it's a jokey name for a famous painting called 'The Birth of Venus'.
Otherwise it's usually a pun or joke relating to oysters (they are served in a half shell)
Another jokey use could be referring to a comic/kid's cartoon called Teenage Ninja/Mutant Hero Turtles whose catchphrase was 'heroes in a half shell! Turtle power!'
Mostly just people fooling around with the language.