Aaron and Peter, thanks for your very informative replies.  I
certainly don't intend to start submitting patches for something as
intuitively complex to implement as this, even without Peter's quick
list of pitfalls.  It helps to know that I am thinking in the right
direction, though.

Peter—
what if the extensions don't register as AutocompleteProviders
themselves, but are offered through an internal provider which knows
about extensions' keywords (or names)?  That'd keep the ranking system
flexible.

Z