Since the usual model is NULL = top level, there isn't a built-in method to bypass that, so you're going to be stuck with custom code and solutions.

If you want to use the 0 or -1 version, you'd just have to create a row with that value, self-reference it, and then ignore it for all practical purposes. Would allow the FK to work. Wouldn't make querying any faster/easier, just would make inserting parent-rows easier, since they'd have a fixed value.