I have seen a Audi Motorsport exhaust manifold from a Sport Quattro, and it has passages and fittings for fule and air injection into the exhaust manifold itself. They did'nt have electronic fuel injection back then
Today all you need to do is to keep the throttle slightly open, pulse the injectors and cut spark. With proper programming, you can do this and still keep exhaust temps within limits even though the manifold ans exhaust housing quicklt will become overheated.