In the past I ran a friction shifter as a drag brake on my old 36er.
I recall that I had to tighten the friction setting pretty tight in order to get it to hold my brake on some of the steeper hills I rode. How tight can you tighten these things?
On my last one, I just ran the shift cable down to my brake through the current brake housing. Is there a better way to do that? Is the shift cable sufficiently strong for long term use? I’m assuming due to the size there is no way to use brake cable in the shifter.
One time a riding buddy had some funny shifting stuff going on with his indexed downtube shifters, and it turned out part of the problem was that he had put a brake cable in in by mistake. So yeah, at least some shifters will take a brake cable. And a few seconds with a drill might fix a lot of the ones that don’t.
And as I recall, you can lock them right up tight if you wind down hard enough on the friction screw. I don’t think that’d be a problem either.
Worst comes to worst, you break a cable and you use the pedals to slow down until you get home and re-engineer it, right? Not a show-stopper.
On a bike I would not use a shifter cable for a brake due to the kind of force applied to a brake. On the other hand, a unicycle doesn’t require nearly the same amount of clamping force to slow, or stop the wheel. With that in mind it might be okay to use a shifter cable. I am using a shifter cable on my 32, and as yet haven’t had a problem.
We used to sell brass barrel ends for brake and shifter cables that needed to be soldered onto the ends. If you could get the right size you might be able to use a brake cable, but with a shifter end on it. The groove in the shifter may need to be enlarged to fit the width of the brake cable.