I'm going to use bushings in the corner channels and
a steel (ideally stainless) hinge pin. The pin won't
wear out, and if the bushings wear they're just a
few minutes to replace.

Anybody up for making replacement pin kits ?
Here's a rough (I know, very rough) sketch:
http://www.nadler.com/Antares/CobraRearDoorHinge_4.pdf

Joe ? Steve ?

Alternatively, can anyone recommend a favorite
internet machine shop ?

Happy Holidays,
Best Regards, Dave "YO electric"

PS: Wear on pins is only on one side: door moving
downwards when closed, or upwards when open. I think
most likely I've caused this by cranking trailer nose
up against door opened on ground to steady rear of
trailer(instead of using the cranky rear jack stands)...