It seems to me this wouldn't carry its weight as an API. It isn't clear that we'd be able to implement it well. People implementing hashable collections would still need to be aware that they need to call this function, so we'll need to call it out somewhere in the docs. But we may just as well describe what they need to do and why.
Not without knowing more details about the collection! In particular, unless we know more about the elements, combining the count seems like a better choice than any constant bit pattern.