I used "welding" because I didn't know there were different ways to "glue" metals. Yes,
Brompton uses brazing instead.
I got a reply from Andrew from Fudge Cycles in London who cold set at least one triangle back in 2009: "We can no longer do that due to health and safety insurance and Bromptons terms and conditions."
There's obviously no way to know how many Brompton users have successfully been riding their cold set bikes for years… and how many broke theirs in the process.
So brazing an extra pair of dropouts might be a safer option… provided it doesn't impact the fold and the ride.