I thought about that. I have a question to a couple of folks who haven't provided an answer.

In the example you posted, the socket diameter is the same end to end. Just eyeballing it, I'd say the length is 20-25mm. My question is that chainstays, and seatstays to an extent, are tapered. Over the length of that socket, the ID of the chainstay increase by 1.0-1.2mm. I have been told when using silver solder, your gaps should not exceed 0.3mm, where the gap at the end of that internal plug will be 0.5mm. I don't know if that is acceptable.

By cutting the stay, the metal can flex over the undercut. At least, that is my theory.