My avatar is at the peak of Mt Washington after that brutal race; there are extended 18% sections. Then there's the wind; 45mph gusts the day I did it. That race requires no brakes as they don't allow you to descend; you have to get a car ride down. Some racers strip their bikes down to a couple low gears, no brakes for a lighter bike. I'm trying to talk my husband into climbing Mt Washington on the tandem to no avail. Way off topic but here's my post about that legendary race:
https://bethsrecipes.wordpress.com/2...ugust-16-2014/ 
I love the hydraulic disc brakes on my mountain bike and wonder how many people put them on tandems. My guess is not many as not many tandems do big hillclimbing.
Just reading reviews on TRP hydraulic brakes - great reviews but cable stretch seems to be an issue causing brake levers to reach handlebars. Seems that would be more of an issue on the tandem with such long cables. ?? Do you ride hydraulic brakes on your tandem?
What about the FSA orbit headset? Anyone with experience with this?
Congratulations on finishing Mt Washington! I might be able to talk my stoker into doing it if we were closer and she lost her mind
Coincidentally, we sold our Co-Motion to a couple on the East coast. Their daughter and son-in-law finished Mt Washington twice on their racing tandem. Can't remember if they rode a Cannondale or Calfee but they're a nationally ranked tandem team.
Cable stretch is not be an issue with TRP Hy/Rd (or any brake) if you use high quality cable and compression-less housing. We use Nokon housing on our Calfee and the Hy/Rd feel close to a full-hydraulic brake. It's a significant improvement over a mechanical brake like the BB7 because it's so easy to dial-in and it's quiet! The Shimano RS685 hydraulic brake would be an ideal choice. It's powerful, requires less hand pressure, and will allow the use of 180mm rotors. It's the same as the R785 but it's packaged with mechanical shifters.
The FSA Orbit should be fine. The cartridge bearings are durable and easily replaced. Frankly, we just use the Chris King because of the bling factor. Allocate your budget to powerful brakes and light wheels if you want to go fast.