Depending on the hopping rate and ABV, you probably won't get much (if any) lactobacillus activity at this point.
Just as well, lacto won't ferment long-chain dextrins.
If you add sugar, brett will ferment it first.
If you add maltodextrin, brett will break it down to sugar, and then ferment it.
If you want acidity, you can blend in some sour beer or add lactic acid directly. A touch of lactic acid is nice because brett can convert it to ethyl lactate.
You could add a mixed culture with pediococcus, but IME pedio in secondary usually has a huge lag time (8 months minimum) and isn't worth it for the small amount of acidity you need in a brett saison.
Good point, Kyle. I forgot that Lacto doesn't handle the long chains. I suppose he could do a small sour mash hit it with. Sacc, then blend back to taste. But that is a bit more work.