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Steve
I think mtn tandems really are a different beast. One I assume you have dual discs and this allows you to use both of them alternatively and powerfully which mitigates the heat buildup to a great degree. Also the speeds in general are not as high. On the descent we melted we had hit a top speed of 49 and then had to slow down to the thirties due to blind corners and poorer road surface. It seems most hills in the US don't have long stretches of 15% grade however in Europe it is pretty common. We have two full suspension mtn tandems with full hydraulics and have some done some significant descents on them here in Alaska and in Moab and the brakes are up to the task. Also on dirt the traction factor comes into play. On a road tandem the traction is so great you can apply the brakes as hard as you can and not skid ( at least with the mechanical discs we all presently use). The failure was sudden when the melting temp was hit however the good part is I still had brakes as it just did it in one area but now pulsed significantly as the disc essentially got narrower in that spot. If I had stopped to allow it to cool when it first started squealing loudly it probably would not have melted. Us and our riding partners are not foolish and frequently stop to allow the brakes to cool however I did not expect the melting on the this descent as it was not that long and we could see the bottom. We both pointed to the road sign at the top that said 15% grade as we started so we were not surprised going into it. the other team that melted had a special machined Saint Ice tech rotor modified to fit a standard 6 bolt hole pattern with the cooling fins ;and they had the same failure and the cooling fins melted also. He is very experienced and we descended Mt Ventoux with them and he only had rim brakes and extra water bottles that they used for cooling the rims as they descended ( his guest stoker was pretty terrified that his job on the way down was to spray the rim!!!). We are a 320 lb team and they are probably a few pounds lighter. We have used Avid, Bengal and now the TRD Hy/ RD for calipers in our quest for the best brake setup. The ICE rotor is probably fine for normal riding but not an advantage then because it it touted to dissipate the heat which really is not a problem in that riding. My recommendation for your event if it starts squealing really loud stop and allow it to cool. I do not know much other then the website about D2R2 i so I am not much help for you.
Mark