Alternatively, what caliper are people using successfully on their drag brake setups? Or Is there a way to put a 5mm brake cable that is long enough through a friction shifter?
I bought an Avid BB7 for mine, I got the one you adjust each pad independently, it works perfectly with a friction thumbshifter.
It was easy to fine tune with the adjusters.
Drag brakes make great parking brakes as well.
Ray