Further comment on my own solution:

The pad on my shoulder belt is one of those wrap-around velcroed ones to reduce pressure on the neck. It was solely for my late wife, the belt rides lower on me, so I don't use it.

The belt itself rides on a track on the door, so the upper end doesn't slide through a bracket or anything. I slid it into the pad, just out of sight, and slid the pad up to the top. The fabric is synthetic and quite strong. Any collision bad enough to rip that loose would probably leave me in no condition to try to get out of the vehicle.

I wouldn't want it down on my shoulder, it would be an almost guaranteed broken clavicle in an accident. I've had a cracked one (along with 8 broken ribs), I see no need to see what a fully broken one is like. If/when I have someone in my life again that uses the pad, I'll find a different solution. I wouldn't try mounting the hammer directly to the belt, for fear of compromising the integrity of the belt.

My solution isn't for anyone who actually uses the pad, but it works for me.