I said "seem" because I have neither conducted bench tests nor can I locate (granted, I haven't tried too hard either) any kind of real world scientific tests (the kinds with controls that reduce variables). But, having a working knowledge of physics and engineering, I can guesstimate pretty well on these things, but nothing I'll dispense a definitive opinion on. Hence my vague reference to physics being physics....slowing/stopping 90kg of mass vs 140-150kg (or more, as you've alluded to) is different, I should have been much clearer on that point.
I am familiar with the kinds of descents you're describing. And, yes, the technique you described (though not what I was thinking) is not possible in that situation. That said, though I am not a tandem rider, but understanding the nature of braking a tandem, the rear wheel is hugely important (as in it is actually critical to stopping the bike). A big finned rotor (203mm) with a big caliper body with finned pads, preferably metallic, will undoubtedly shed heat better than a small rotor, no fins, organic pads, small caliper body. Again, the problem is that the component manufacturers aren't testing these setups, and I think that is where the reticence/refusal to warrant them for tandems comes from. It's unfortunate, but the reality is that in the modern litigious society we live in that is also dominated by huge conglomerates that ONLY care about quarterly profit statements, the tandem market is small enough for them to regard it as niche and not worth their time, which is maddening. But, hey, I feel for you, because I bought a bike 5 years ago that is amazing to ride, but is obsolete (post mounts not flat mounts) and then the industry changed, and then I decided to use MTB calipers and everyone said it wasn't recommended...but some common sense and experience with brake setups, and basic understanding of engineering and physics, I knew it would work. Sorry to ramble, I guess my real point here his that even if Hope or Shimano or Whomever won't warrant a particular setup, with some careful research a satisfactory solution can be had. Moreover, if a company will warrant a setup in one country, but not another, that is not indicative of something not working, it is indicative of the nature of warranty law, which is a subject I know nothing about, except that buying a bike in Europe does not mean it will be covered in the US.