Remaking the quill part longer wouldn't be difficult. I'd make it in 4130 steel to be safe. Heavier than your alloy part but let's face it this is not a weight-weenie bike. You could re-use the existing wedges, just cut the angles on the steel tube to match, and get a longer bolt. A plain single-diameter (7/8") tube without that larger diameter ring would be fine I'm sure. Ok, well that is a safety-critical part and it's your neck I'm risking not mine, but I would ride it for sure. I don't think the larger diameter ring is doing anything.

Cover the exposed steel with some aluminum — 6061 tube, 1" x .058", cheap at Online Metals or ebay. Mostly for looks, so you don't have to get the steel quill painted or plated. It won't add strength, but is weighs practically nothing and can be polished up nice.

Hit me up offline if you want me to make it for you, the quill and/or the alloy cover. Not cheap compared to some 'Hissing Lung' doodad from Asia (if such a thing exists), but not too expensive I think. I'd just use a hacksaw to make the angled cuts on the steel tube, which anyone can do if you have a vise, saw and a way to clean up the saw marks (belt-sander or file). Match the wedge angles by trial and error. I'd cut the Al cover tube on the lathe to make the cuts real flat and square, but since it's not structural, it doesn't really matter — hacksaw cuts cleaned up with a file would suffice there too.

Bravo on the bike, the build is superb.

Mark B in Seattle