You are aware of the LuceneSearch extension used by Wikipedia, right? Not that there's anything wrong with making a Sphinx plugin too, but your introductory preamble on the linked page and your apparent uncertainty as to how to proceed in replacing the default search suggest to me that you might not be aware of it to begin with.
Incidentally, there's no mileage in having access to the default search. It's slow to even maintain the indexes. Anyone using a better search solution would be best off dropping the relevant indexes/tables or otherwise disabling it for performance reasons if nothing else.