> I'm fine with adding a note to the Python 2 docs, but putting it in Python 3 seems to be going in the wrong direction as having sys.meta_path not be empty is how Python will be going forward.

I don't think putting a note is any hint that the Python 3 behaviour (which I appreciate) would change, it would simply tell the reader that the list is not empty by default and they ought decide whether they want their finders to take precedence over (and insert at head) or be fallback to (and append) the builtin finders.

It could also link to the standard/builtin finders.