JPK-

On the question about rib-solder work being enough to do the trick on TIG weld HAZ...

In that bit of work it's important to get the work up to temperature all the way around the barrel (equal treatment of HAZ), everywhere there's HAZ, all to the same relative effect.
Speaking as someone who's put down more pounds of silver into brazed joints than I could ever possibly recall, and likely factors of magnitude more of soft solder in joints similar to rib joints, I wouldn't want to have to try to anneal that kind of weld *and* attempt a rib solder job at the same time, given my understanding and respect for issues of HAZ related stress/relief.

For the sake of a complete answer, I will say that it's likely possible, perhaps easiest in a hearth type arrangement where the whole barrel set is immersed in an controlled temperature open oven of sorts.

In the context of the droplock seen above, I would worry about the integrity of the shoe lump joints during that kind of excursion.
Also I'd be concerned about whatever was used on the male/female fit of the sleeve job.

I simply do not feel that there is an elegant solution to the appearance of the joints of those WR barrel sets.

Also, see Bramble's comments above.

Huvius-

Notice the specific language I brought to condition in my hypothetical comparison of old vs new H&H.
As-new, perfect condition, 'Sleeping Beauty' safe-queen turn-of-19/20th Century Hollands give me the willies.
There's been more than a handful of them that once fit that description until they got woken up again in the last ten or fifteen years that 'mysteriously' popped their barrels during load development, while in the hands of highly experienced double rifle owner/shooter/handlers using known-good load recipes and components.

There seems to be something about that combination, that's been kicked around in numerous threads here and elsewhere.
Much more to it than simply vintage vs modern barrel steel.

For as-new condition give me new gear made of recently alloyed modern material.
Otherwise I want to know it's got the nuts to handle the work and has a track history of doing it well.

Cheers
Tinker