Hmmm....

The dropout issue has already been raised. An aluminum track frame will give you weight very similar to the "unobtainium" you lust after though there are titanium and probably CF track frames out there as well.

Are you sure the modern alloy shelled Nexus weighs 2X as much as a vintage Sturmey Archer? If you're going modern you might as well go all the way. That and I'd rather have the Nexus roller brake than a true coaster, especially if I'm running skinny high pressure tires.

Actually, I'd rather have calipers than either but if you want a pedal activayed brake I'd go roller.

I've often thought of this as well. In fact I have a bike with a vintage 531 frame that might lose its derailleurs someday. I lean towards doing it with a steel frame but if you go modern frame you'll have the lightest internal gear roadie around.