I'm sure you're all about to wet yourselves with excitement - here's the conclusion.
First up, a shot of the lug/sleeve (let's just say "slug") installed on the seat tube and cleaned up. Mmm. Pointy.
Next up, I go ahead and tack the toptube in place, after positioning the slug where I want it. Note that I don't braze the slug in beforehand, as the flux and goop makes welding difficult and unpleasant, if not impossible (another lesson from the school of hard knocks, that). Here's a shot of the slug with the toptube tacked to it.
Finally, I weld up the frame as normal, then go back and flow a little 56% silver into the top and bottom of the slug, simply to keep any air pockets between the slug and the seat tube from messing up the powdercoat. The slug doesn't need this for structural reasons - the toptube and seatstay welds hold it in place just fine.