Had another look and I reckon there was about 8mm of braze along the top edge, and much more than that on the bottom. So I think that was a good joint. Then I noticed a bit of filler rod in the seatstay:
So I welded it to some bits of scrap tubing, badly and in the wrong order, and inhaled a bit of burning paint, so I could get enough of a grip on it to do some destructive testing with a swing press:
As you can see, this one did fail at the braze, but not before the seatstay I was bashing on was fairly crushed. An acceptably strong joint for a dropout I would say although I would be disappointed if this were between main tubes. The other side:
The actual seatstay is ripping apart but you can also see that the braze has broken. After then pulling off what was left of the seatstay:
And, finally, a closer look at that strange bit of rod left inside:
I guess he must have been feeding it in through the SS, at this point not attached to the bike, and then it got stuck so he cut it off as far back as he could (I don't know where that was, the cut you can see is where I lopped the SS off ages ago and I didn't notice the rod then).