I've given this fix a lot of thought. I'm going to join a piece of seat tube to the top of the seat tube that is left. The problem is have it align after with almost exacting precision. I figure if I align the tubes with a seat post, then clamp it together in my jig, then clamp the ends of the tubes with c-clamps, then tack it in 4 places, then tin it after I might be real close. Call me Jig Clampet.
This is what I got right out of the jig. Not too bad. It's going to be covered by the sleeve anyway. The important thing here is alignment.
With a little filing and sanding I have a smooth seam. and the post slid in and out just fine. I thought I'd be a little off, but it work perfectly.
The trick is to use brass because it's got a higher melting point than silver.
If I had joined it with silver, I would risk melting the seam when I sweat the sleeve on with silver.