Injecting after brining might be "gilding the lily", so to speak. Brining will add flavor to the meat and help it retain moisture. If you do inject after brining, I'd be careful about the amount of salt in the injection liquid, since the brine will have already accomplished that. And I'd be looking at flavors that reinforce or compliment the flavors that were in the brine, or ingredients that can't be accomplished in a brine (e.g. warm unsalted butter).