Heh, I did the same thing with a disappointing holiday ale a few months ago; I'll tell ya how it came out in a couple months!
At least in terms of getting the flavors you're looking for, you probably don't need to add any sugar; 1.021 will leave plenty for the bugs to chew on. Ditto on the champagne yeast; bugs work slow, so it might take 2x-3x as long, but they will carb the beer. However, your best bet is probably to let the bugs chew on the residual sugars for a few months, then once the gravity's stabilized again, add your simple priming sugar of choice as if you're bottle carbing a clean beer.
Finally, if you're looking for sour rather than just funky, you're going to need more than just brett (although, you'll want the brett, too, to clean up after the pedio). You can get a vial of sour blend, or just grow up a starter from dregs of one or more favorite commercial sours.