I would say any modell would work, so you could use e.g. something from shimano or Avid.
But why would you want a V-Brake if you allready have a very strong disc brake?

Oh, okay. I actually never had a disc fail (and really hope it will never happen), but a V-Brake fail. Anyway, if you want a backup, it makes sense.

I like the backup idea.

Makes very good sense to me.

After destroying two brake discs on long descends with a heavy loaded Pino I also added a V-Brake, which is controlled by a bar-end shifter. This allows to use it as a "drag brake" on longer downhills or as a parking brake. I simply soldered some cantilever bosses to the frame. We have a heavy 26"-rim on the back wheel, so it can handle same brake heat.
[Blocked Image: http://velo.dyndns.eu/bilder/pino-zusatzbremse1.jpg]
[Blocked Image: http://velo.dyndns.eu/bilder/pino-zusatzbremse2.jpg]
I would never even think about adding an additional rim brake to the front wheel. The 20"-rim would heat to quick, if nameable brake power is generated there. I once witnessed a tire defect on a long descend on a 28" touring bike caused by an overheated rim. A 20" rim on a tandem is much easier to overheat. If you don't want to solder/weld something to the frame, then there are some adapter systems to retrofit canti sockets.

Our V-Brake is operated by the stoker. This makes it easy to use and place the lever, but modulation in combination with the disc brake is a problem. Braking too hard on the back wheel is not a big problem - braking (unexpected) too hard on the front wheel will be dangerous.

Andreas