I saw on a site, don't recall if it was loosescrews.com, that carried a kit to bolt DT shifter bosses to a frame tube or stem.
It uses a long thru bolt. I'm sure the symmetric body could be modified or a machinist could mill a body to accept this kit.
I believe you're talking about this:
http://www.cycle-frames.com/bicycle-...-AL-FRAME.html
Though that is interesting and loosely relevant, I'd submit that it would require excessive modification and machining to put it mildly. That kit is really intended for aluminum frame-building.
The downtube adapter that I make (4 photos posted above) is an off-the-shelf solution that would allow immediate downtube shifter installation without modification of the frame. My design works with off-round tubing such as tear-drop, oval, and early 1980s "aero" frames as well.
As it happens, I've got a Le Tour Luxe frame under my roof, so I can check the exact fit and report back later today.