Your picture shows pretty much the sort of thing I was planning Doug, but I was going to separate the two ends of the sleeve as I have a long head tube and don't want to add unnecessary weight.

However putting the sleeves on the TT and DT as you suggest and then filleting to the head tube means when filing the fillet the danger of over thinning the tubes is eliminated - a very attractive advantage which pretty much wipes out all my doubts about fillet brazing (see below) - which is a shame as that head tube looks the Dogs Bo##ox and I would love to do something similar. Using this method, could you silver the sleeves and then do the fillet in brass? I know some of the silver would melt but would this be a problem?

After fillet brazing my first frame (Reynolds 631) I started another in Columbus tubing, but scrapped it after finishing the front triangle as I was worried I had over thinned when filing the fillets. The Columbus tubing was much softer than the air hardening 631 had been.

Now I know it is called bilam construction I will do some googling.