As per my reply in that previous thread, I strongly suggest you take your broken tailpiece to someone with a laser welder. I had a vintage National brass (nickel-plated) and a NRP modern brass (nickel-plated) tailpiece both repaired by a laser-welder, in a dental lab.
For the minimum damage to the nickel-plating adjacent to the fracture line, a laser welder causes the least cosmetic damage AND the strongest join. Both halves of the tailpiece will join to form one continuous piece of metal again, rather than 2 pieces with a different material in between (eg. Solder).
I just took new pictures of the tailpiece on my 1930 National Style O. It was repaired just over 5yrs ago, and has been permanently strung up to full pitch regular E tuning, and swapped back between Open D, Open G, Dropped D, Open D Minor, and back to regular E tuning, multiple times over the 5 years, ALWAYS with 13-56 strings on the guitar.
The tailpiece is as strong as when it was repaired over 5yrs ago. In close-up pictures, yes, you can see the join BUT any other method causes worse cosmetic damage. See my pics taken today :
NB. This is how it looked immediately after the laser welding. No attempt was made to grind, smooth or polish the repaired join, for fear of removing nickel plating. It’s pretty neat, I think, compared to brazing and welding examples I have seen in the past.
I do think the tailpiece will be just as strong as before, if repaired by a laser welder. At the thickness (thin) it is, it has certainly lasted a long time so far, so I think it would last well again repaired this way.
Anywhere a workshop/lab might be that processes metal, makes metal thing, repairs metal things, MIGHT have a laser welder. They are expensive so not all places will have one BUT a jeweller, for example, might use services of someone with a laser-welder, so you could ask around.
When I got my dental lab to repair my two tailpieces (I’m a Dentist, and use this lab often), I was buying a 3D scanner from them (for 3D scanning patient’s teeth, to make 3D printed models of their mouth, and design & make crowns&dentures) - for £27k !!!! So, I thought THEN was a good rime to ask them a favour and repair my tailpieces !!!!! I brought my 1930 Style O with me, to show them - and they were so fascinated with the guitar, they all stopped what they were doing to have a look. I’d imagine someone will be equally fascinated with your 12-string guitar.
The lab technician who operates the laser welder was concerned that it won’t be possible polish the repair after welding, for fear of removing more nickel-plating BUT he was confident there would be little loss of any plating close to the repaired edge.
I brought a National tailpiece from another National to show what it should look like, angle wise, once repaired. The lab mixed a large amount of silicone “lab putty”, and pressed the spare, unbroken tailpiece onto the putty before it set.
Once set, the tailpiece was removed, creating a jig/ mould for the two halves of the broken tailpiece to be accurately aligned. You can see in the pictures the small and only mild burn marks to the silicone putty, caused by heat from laser welding. The heat produced (enough to melt the metal, and allow it to fuse to the other broken half) occurs in such a small area, for such a short split of a second, that there is little transfer of heat beyond. It takes lots of individual spots of laser welding to repair along the length of repair line.
If there any gaps between the two parts to be joined, a thin bit of brass can be inserted into the gap - and laser-welded on each side - and it will all fuse together as one piece of metal.
All the best - and do let us know if & when you get your tailpiece repaired.
I could always ask my lab to repair it for you ? I’m in Scotland.
NB. The last photo shows my repaired NRP tailpiece. I got them to try repairing that one first - to let them “practice” on that one, before having a go on my vintage National tailpiece. They made a better job on the vintage tailpiece, cosmetically.
I have seen repaired tailpieces done by National Resophonic. One I saw was laser welded, polished, then nickel plated again. An outstanding result. Perhaps contact National Resophonic. The cost, including shipping both ways, might be high though.