According to zxcvbob :
There's not an NEC compliant way to do it. If you want to do it safely
but not compliant, do what you said but then silver-solder the
connection and wrap with rubber splicing tape. Regular solder isn't
good enough, you'd have to either weld it (and copper is hard to weld)
or use silver solder with a high silver content (like 40%.) It would be
a real pain-in-the-ass to do, and a half-assed job when you were done. HTH

I'm not recommending this:

Was helping a friend rewire a house (almost complete wall teardown and removal
of K&T), and we were left with a beautiful plastered ceiling we didn't want
to touch, and we needed some way to reattach the K&T to the ceiling fixture
to the new wiring, and it was in a real bad place to put an accessible box.

So we held off on that, with the hope that the inspector would have a good
idea on his next visit.

The inspector told us to twist the wires together with at least 2" or so of twist,
solder (with ordinary solder), tape thoroughly, and "don't let me see it".

This is basically replicating K&T connection techniques.

[K&T was mid-air spliced all the time.]

That's not legal. But the inspector told us to do it...
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.