I can't say I've ever really noticed the word Hoss. Really, 100+ time in the stand? I read that recently (complete and uncut 1990 version) and can't really remember coming across the word.
I'm not too sure about words - I know in his Maine books alot of people say Ayuh, but I have noticed 2 themes in most of his books I have read:
1, People always have dreamless sleeps in his books, usually after an event that would likely cause a restless night.
2, Rather more disturbingly, Stephen seems to love writing his characters wetting themselves.
My first answer was going to be that "hoss" made it into the vernacular with the advent and popularity of the TV show Bonanza, which SK might have watched as a kid. He would have been 12 years old when the show premiered in 1959. Often, words and phrases from televisions shows we watched in our youth make it into our adult vocabulary.
Kinda like the kid (Seth) from the Regulators liked some of them old shows. I don't remember if Hoss was one of the words he would say, but I'd guess it was...