UPDATE

In the end I just fitted a domed washer inside the rim, pulling it tight into the dip with a nipple threaded through it and tightened to the end of a spoke.

I domed the washer by fitting it over the conical head of one of my tin bashing hammers in lieu of a mandrel, put the washer on the top of a vice with the cone between the sides, then tapping the back of the hammer with another.

The great thing is, no brazing or welding needed.

I took it along to a mate who is a motorbike mechanic, and he inspected the repair before putting the tyre on. He said it was a good repair.