I believe the original Tales of the Miskatonic Valley includes a Scenario with a Serpent Person.
Also, here is a like to a Starry Wisdom article on Serpent People, I think the "Call of Cthulhu Adventures" section at the end is Scenario seeds rather than references to published adventures.
https://web.archive.org/web/20091105202308/http://www.chaosium.com/chaosium/starry-wisdom/sw3-racesp.shtml