Mike could you clarify that you are wanting sleeves on both the head/seat tubes as well as on the ends of the top and down tube to create what looks like a lug? This was done by Claude Butler after WWII when lugs were scarce because industry had all been converted over to make stuff for the war effort. It has been common to do this bilaminate construction ever since.
There are several ways to create a bilam construction but I think the easiest is to carve the shape on the sleeve and then braze the sleeves onto the top and down tube and then miter the unit together ready to fillet braze those tubes to the head and seat tube.
It is also possible to do a half bilam construction where one sleeve is brazed only on one of the 2 tubes at a junction. Here is a picture of a frame one of my frame building class students made that required a sloping top tube. The cut sleeve on the head tube gave it distinction.
