Do mechanical disc brakes suffer from heat-induced fade?
From my experience, the answer is no. I have been cycling across the Great Glen and have no problem with mechanical disc brake.

My bike was moderately loaded with camping equipment. So the load (including me 70 kg) was around 110 kg or 242 lbs. There were continuous downhill sections but the brake works fine, thankfully. 



From a scientific stand-point, there are three types of fade


Green fade - caused by resin boiling off of new pads
Fluid fade - brake fluid boiling and system losing pressure
Pad fade - pad overheats and loses friction


Ignoring 1, 3rd cause starts to occur at much higher temperature and require time than 2nd cause (which occurs at around 120 oC for moderately contaminated mineral oil*). That is why you need to check your mineral oil before setting out on a trip.

Pad fade is the main factor in Mechanical Disc brake fade. Unless you are not braking heavily and continuously for more than 10-20 minutes, you are fine.
In number, if you are going down 20% gradient hill, braking continuously to keep your constant speed at 18 kmph (11 mph), the braking power is around 1078 Watts for 110 kg (bike+load+rider). That means, referring to **, you can safely brake
for hours, if not hundred of hours for the iceTech.

Refer

*http://www.epicbleedsolutions.com/blog/dot-brake-fluid-vs-mineral-oil/images/shell-dot-fluid-boiling-points.png

**http://cycle.shimano-eu.com/publish/content/global_cycle/en/nl/index/news_and_info/Velotech_Disc_Brake_Test.html