Gard-
...enjoy that challenge, bro!
Most of my favorite things in that originals' band was when the saxist, drummer, & I did our thing(the guitarist usually got lost after about a minute).
I wouldn't call our ramblings "Free Jazz", either...
my situation is about the same as your's(groove player NOT a Jazzer).
It's nice not having a chording instrument getting in the way!
BTW, I can't believe this is a 'paying gig' AND it includes an audience?!
My only suggestion-
...I really listened to the drummer(feel changes, time changes, dynamics, etc); basically, the sax guy did his thing.
Also, if you lock it down with an ostinato(say, a 2 bar phrase), think about moving the ostinato around within the bars(displacement)...or if it's in 4, do what it takes to put it in, say, 5?
Have fun!
Oh yeah-
Check out Roy Campbell's Pyramid Trio(Campbell on trumpet, William Parker on bass, & Hamid Drake on drums). Also, sometimes, Miles & Carter & Williams do it together(Hancock lays out). There's possibilities to be had...
