It can be done with common rods too. The 333 is just a particularly specialized rod for the technique. I run a brushful of anti-splatter gel into any threads exposed to serve the same purpose as the 333's fancy flux. Horizontal makes no difference with a quick freezing rod. The whole thing works because as the heat sinks down the restant it makes it expand. It wants to expand in all directions, but it can't because it's width is constrained by the walls, making all expansion go lengthwise. Then it cools and shrinks ... but it shrinks in all directions ... leaving a piece that's now slightly longer, and slightly skinnier than before. The added materiel and welded on nut are little more than a handle at this point. Run a thread chaser or tap down the hole to clean up any distortions and you're back in business. A practiced hand at it can clear the hole in under 10 minutes.
I will say, I've never attempted this on an aluminium head.