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an update, if anybody is interested in my little sicheeation...

Decided to try the easyout route, mostly due to the hassle of towing the whole rig to a local machine shop. Jury is still out as to whether that was a good decision:
Enough of the nub (7/16 SS) protruding to allow me to smooth it out with the Dremel.
Eyeballed the center and marked it with a Sharpie marker. Hit that dot with a center punch good and hard. Started with a 7/64 Cobalt bit. Worked very slowly, but got it all the way through the bolt, a little over an inch. Good news, not a blind hole. By my eye, close to down the middle. Gradually reamed that out to 1/4" with progressively larger bits. No problem, but I went very slowly with the 1/4 bit, constantly checking my center, hoping to avoid breaking through the wall. Looks good all the way through the hole. Shot PB Blaster in the hole and let it soak a bit, at last getting some help from gravity. Tried a couple of times heating the aluminum casting with the propane torch, but still no go with some gingerly attempts on the #4 easy out. Thought I might get lucky. Shot in a bunch more Blaster in the hole, hopefully creating a little pool around the threaded hole in the casting. She's going to sit for a few days, probably a week until I get to it. Am wondering if chucking an oversized bit and running it in reverse with the hammer drill will create enough vibration to help break the surface tension, more than just a hammer blow. Didn't want to risk breaking/dulling the bit by using the hammer drill while drilling the hole. More torch time is in my future.