Regarding your comment (and picture above) about the qr on an adjustable stem....
I'm doing something similar on my Birdy, except I'm not retrofitting a qr... I'm using a stem that is already built to do this!
The Satori STquick release quill stem allows 90 degrees of adjustment and locks in place, rock solid, using the quick release lever that has ridges in the hinge.
Here are pictures I took this weekend of my test fitment on my Birdy.
With the stem qr lever open, I'm able to swing the bars vertically, which then allows the whole thing (steering tube, stem and bars) to fold flat against the bike when I fold the whole thing up without any worry of the bullhorn bars sticking out, or without worry of having to be spun around to go through the spokes and frame.
The lever locks into place with that little red button. You can't open the lever without first pressing in the red button.
The dial on top of the stem adjusts the tension of the qr lever.....