The 1992 Trek catalog doesn't say what alloy the stays are made from, but it doesn't matter that much on a non structural component like this. 5356 filler will work with 7005 and 6061 alloys. The weld will almost certainly melt into the threaded hole, so it will need to be cleaned up and re-tapped. It's going to be tough to weld that without having it melt away. If a stainless M10 bolt could be threaded into the hanger, that would go a long way toward preventing damage to the threads and acting as a heat sink to prevent it from melting away.