Ok, I've got a question here... right now I have a hardtail that I have used for light touring (350 mi), and I am almost daily taking my kids on their Piccolo trailerbike down and back up the very large/steep hill that I live on. In 3 years, I have managed to blow through 4 bottom brackets, 3 rear wheels, 2 pedals, and a partridge in a pear tree. To say I'm a 'hard rider' is an understatement.

I am a heavier rider to begin with (230), and plan on building a touring bike piecemeal. I am taking notes from the tandem guys, and already know that I am going to have a 40-spoke rear wheel built. But I'm wondering about the braking system. I keep my hardtail V-brakes meticulously maintained, but still have noticed the fade in braking power traveling downhill under load. For my next serious tourer, I was wondering if I can do something like what they do on tandems with a third brake?

In particular, I was thinking about building a touring bike with cantis, bu I was wondering if I could add a mechanical disc brake from a friction thumb shifter... Ideally, I could effectively use an interrupter brake on top, while simultaneously creating drag on the thumb shifter/disc brake mounted upside-down under the same bar.

Is this crazy? Is there enough mechanical advantage in a thumby to use it as a third brake?