I can't tell from the pictures, is the butt just rounded, or was a piece actually missing?

If you are just building out the square, I see no reason the JB wouldn't work. I might prefer Accra Glass gel with the powdered steel mixed in, but that's just me. If you are regluing a missing piece, it probably would still work, maybe need a little creative reinforcing, but as others said it's a low stress area.