That's more or less what I've been thinking about doing, although I'm using a twist-grip 7-speed shifter for a derailer set up. I was going to mount it where downtube shifters normally would be, on a modified Varsity. However, I've been trying to wrap my head around how do it with a bolt-on mount, I don't want to weld to the frame. But I was envisioning a section of handlebar cut with a fishmouth to butt up against the frame, and then have some sort of clamp around it to attach it. Or possibly modifying a threadless stem with a 1" clamp diameter (since that would fit the frame tubing), and turn down the neck of it to a handlebar diameter...