I think your on the right track, go the bolt and a rubber gasket of something ?, and you will be ok. maybe even a o-ring under the bolt or nut on the brake side ?, worth a shot. I don't ride in water ( not unless I have too ?..lol ), but if your on dry land ?, don't worry about water getting in.The brake drum/axle holder has a blind hole that this bolt inserts. I was thinking I could either buy a time-sert repair ($$) or just open the end of the bolt hole and pass a nut/bolt combination.
This does open into the brake shoe space. I didn't know for sure if this needs to remain a blind bolt hole for keeping water out? Again, I was thinking a rubber gasket around the outside to keep water from getting in?