"in a Folk Context I'm aware that Improv is a no-no"

I think that sort of context is very rare here in the USA. For one thing, there are so many traditions that it's impossible to tell when someone is being traditional and when they're taking freedom with a form. Shape note singers have to stick to the book, I guess. For many cultures here I really have no idea if they're being traditional or merging styles. We have everything from Mexican mariachi to Polish polkas, with new ones coming every year. Ballad singers get to choose from English or Appalachian versions. Sea chanties can come from traditions from Newfoundland to Jamaica. And that only brings to mind a new category :::shudder::: World Music - which I can't define and somehow seems to be misused a lot.