To me, this is where the fun starts. Depending on the accompanist, and the
flexibility of the accompanee, you can really do anything that doesnt clash
with the melody note, or even that does (why not a Bb Locrian, or a D6 over
Bb to Absus/Bb, and up to G6/B. I mean, it might get you fired in some
situations, but Herbie wouldn't have flinched. The real answer is that
great accompanists have many many arrows in their quiver, and can play it
as it lays to make their partners (and the music) sound great.
I say screw around to the max in the privacy of your own practice room, but
a-9 to D7aug is the way to go in public.
Eric