The hinge: it is a 1/4" pin welded to one of the exterior plates. On the next one, I plan to weld it to both exterior plates. The ends of the pin and the holes in the plates have a chamfer, creating a lot of volume for the welds.
Once the welds are ground down and painted, the pin is invisible.
Malcom,
Your idea is intriguing. I am also concerned about hitting the paint with a hard steel part. I was planning to use some kind of rubber sleeve on the bottom part of the lock, but your approach would solve this problem quite elegantly.
I guess I'm heading back to the drawing board (well CAD software, really)...