The saison is already fermented out, so there's nothing for the brett to eat. I'm adding Dextrin so the brett will break down those long dextrin chains to continue with the fermentation...the reason I'm going this route is because I mashed at a low temp range to make an attenuative, low dextrin wort, for purposes of making the saison. The souring is an experiment that I'm doing on 5 gallons of the split 10 gallon batch.
Brett will create flavors without fermenting sugar/dextrin. Actually, if you add sugar/dextrin, brett will make more alcohol, which will distort the side by side tasting at the end of the experiment.
Brett will metabolize other compounds besides sugars to create different flavors (acids, hop compounds, yeast esters, etc etc). Even at 1.000 FG, brett will continue to change the flavor of the beer.
Starch/dextrin are important in lambic because of the long lag time of pediococcus. Pedio has the ability to break down starch/dextrin, but it can take 8-12 months to reach ample cell counts and start producing acid. Brett works in concert w/ pedio, reducing diacetyl and turning lactic acid into ethyl lactate (pineapple-peach ester).
The wonderful thing about making a mixed-culture saison is bearing witness to its evolution. It is fantastic fresh and fantastic with age, evolving into different beers in different stages. Not only is adding additional fermentables unnecessary, it (IMO) takes away from this experience.
Trust me on this - just pitch your mixed culture and let 'er go for awhile.