This is an image that appears to be one of the original drawings for the books:
But I don't recall Snape ever being mentioned to have facial hair in the books or otherwise.
Has JK Rowling ever implied Snape as having facial hair, in any medium?
Prior to the film being cast, JKR drew a number of pen+ink pictures of Snape. Some had facial hair, others did not.
No. Rowling has NEVER mentioned Snape having facial hair in any text.
Mary GrandPre has said in interviews that she and Rowling do not collaborate on the illustrations:
I don't talk with J.K. Rowling. I deal with the art director at Scholastic, David Saylor, when I work on the books, which is common: to keep the illustrator and author separate.
Technically, they aren't really canon... And GrandPre only illustrates the books in the United States. Her art is not included in the UK edition of the books.