OP here:
After considering all the good tips offered here, I've decided to proceed with my JB Weld idea.
1. It's machinable for milling and or sawing afterwards.
2. All the surface area of all the threads of each of the 9/16in bolts should provide plenty of contact area and strength for this application. New subsequent 5/16in tapped through holes on the perimeter of welded caps might get a little "rattely".
3. Invisibility of repair is not a giant concern to me.
4. 500degF heat rating is plenty good enough.
5. The only downside I can think of is that JB Weld is kinda thin and it tends to run a little, and will tend to "drain" away from the bolt threads until cured. I may look for a means to increase the viscosity, or seal around the bolts with play dough to prevent this. Read about that trick on "another" popular forum.
6. Never had good luck with the rapid curing type so I'll stick with "standard" JB.
I considered a 3M industrial epoxy product called 2216, also known as "blade glue" is sometimes used to help hold helicopter blades on, but my stash is about 10 years past the shelf life.
Stay tuned campers.
