I wonder what it is about the Bay Area that causes a simple disc brake on a road bike to be so problematic, whereas those of us using them in the Southeast mountains of North Georgia, Eastern Tennessee, and Western North Carolina have no such problems? Seriously, I can't ever recall bending a rotor on my road or even off-road tandems... despite more than our fair share of off-road crashes that have clearly put the rotors up against mother nature's out croppings and such. As for finicky set-up, all I've eve done to my Avid is to add a compression spring to preload the brake cable which improved brake lever feel (by eliminating slack) and eliminated rotor drag by making sure the caliper's pads were always returned to a fully-open position. Well, that and I believe the S&S cable splitter sitting 1/2 way down the brake cable run may also mitigate some of the slop in an unbroken, 6' long brake cable. I've stuck these compression springs to most all of our friends disc-equipped tandems... and there are quite a few of them: no disc brake related problems to speak of in several years.

Regardless, you'll need to take your cues from other tandem teams as to the threshold for rim brake performance on your local mountains. I've cooked my rear Deep-V rims enough to make them hot to the touch -- which was precipitated by heavy outgassing and brake fade -- but without any dire consequences. Before adopting a full-time rear disc I made a point of biasing my brake usage front to back so that each rim / brake would get a chance to cool off vs. applying both brakes simultaneously. I also was mindful of using the front brake as much as the rear so as not to over use the rear... something that I think many cyclists and tandem teams in particular may be prone to do.

As for your set up, the 28mm tires should afford you better heat capacity vs. the 23mm and 25mm tires that we run. However, I would expect that the slime tube and kevlar tire liner's will contribute to increased tire heating from friction -- emembering that the "brake" heat we are all concerned with doesn't only come from the friction between the brake pads and rim, but from the friction between the tire and the road -- so I am inlined to ask if they are really needed or if you're just applying the old "belt and suspenders" to prevent potential tire flats

Anyway, you'll have to decide if the drum brake will afford you piece of mind: no one can really tell you that you don't need one without spending a lot of time riding with you and observing how you use your brakes and the type of descents you're encountering.

I still remain a big fan of the rear disc brakes and even went so far to have one of our tandems that already had an old Hope mechanical disc drag brake retrofitted to accommodate the Avid. It was money well spent. I only mention this because I seem to recall that your '04 Co-Motion was a Supremo (steel) which makes it a prime candidate for a retrofit.

In closing, hear everyone out on their experiences and be attentive to your brake performance on those regular mountain descents. Although we've never experienced one, in talking or reading the accounts of those who have had brake-induced heat failures, heavy brake usage to the point where the brake pads begin to outgas, glaze, and generate both audible and tactile signs in advance of brake fade... and that's when rim heating really soars.