I think that all of us have experience with various forms of brakes on a tandem. Most of us have experience with only one type of brake. My question had more to do with an objective comparison between rim and disc brakes under the most demanding conditions. I doubt that such a comparison exists. Short of that, the best comparison might come from a bike brake engineer who truly understands the trade-offs between the two. I'm sure that the heat transfer dynamics are extremely complex.
However, because rim and disc brakes are so different, I would bet that one of them would be the clear winner under such conditions.
There is almost no doubt that disc brakes perform better in wet conditions but we almost never ride in the wet so that is not a concern for us.