You are more elaborate with casting the eye. I had in mind simply embedding something like a cup hook into the molten lead plug. 1/4 npt is a good size and easy to get 1/4 pipe couplings and cut in half to braze or weld onto the exhaust. You need more contact than just the thickness of the exhaust pipe itself. A coupling can be slit down the sides with hack saw or zip disc to create a workable die to thread the lead slug.
Sounds okay, but I think that will be slower to release than what I want personally. The bigger the plug the slower the melt in an overheat situation.
I was thinking maybe a #10 dia tapped hole in a sch. 40 1-1/2" dia water pipe exhaust. That will thread and hold fine for this purpose.
Re wheel weight lead -- the fusing temp can be checked when melting before the pour, which I'll do.with a non contact thermometer. As a guess, the antimony may not be a problem from a practical standpoint anyway -- remember the exhaust pipe itself is going to be lower temp than the EGT since it's air cooled on one side and to release we have to melt out the full thread length of the plug in the pipe. It's a heat sink for the fuse.
Ideally we'd rivet the fuse in instead of thread it in, then we'd just be melting out the internal rivet head for a release. But riveting means we have to disassemble the pipe to renew the fuse.
Luckily 620F (or less for an alloy) is a fair way down from 1000F, which was the stated limit, earlier in the thread. So I'm guessing a small gage lead screw will melt out in time. Not sure where that 1000F figure came from for a Lister limit, but sounds reasonable as a ballpark.
I'm sure there will be plenty of creativity around this, and whtever the form it takes, anything is better than the nothing most of us have now for OT shutdown capability.