You would need to pre-heat and use a hi nickel rod and post heat. Many off roaders have their tubes welded to the front pumpkin, including the custom high-pinion Ford front axle in my Jeep.
Metalcloak makes robust covers. I'd bet you could drill and tap the 1/8" npt instead of the bung.
My recommendation - drill and tap the axle housing itself instead of the cover. The main purpose of the beefy covers is to protect the axle.
It doesn't seem like having a sensor where it can get bashed is the best way to go.
I would not drill and tap the axle housing, it would nearly require a complete tear down to get all of the crud out of it. The sensor will be pretty high on the cover and I do not rock crawl so no worries there. I may just drill and tap the cover, now that I have received one, it is at least 1/2" thick if not thicker.