Until those companies start actively heat testing their epoxies and posting the results, I'd not recommend them for somebody that lives in the mountains.
There is definitely testing that goes on with the resins and epoxies. Just recently the rims we are using have gone through a change to use better resins and also a 3K weave on the brake track. That has helped significantly and I can tell you personally we have only had one rim warp from heat. It was following the USPro road race that we just had here in Greenville. A spectator was following a car going down the 2.2 mile hill. Because there was people everywhere the car kept it under 15mph the whole way down the hill with my customer stuck behind the car. 10 minutes of continuous braking later on a hot day and that was enough to warp a rim. I am thinking it would be enough to make any carbon rim meet their doom.
Keep in mind the manufacturer that is making our rims is also doing it for some of the largest companies out there. It's not some fly by night Chinese manufacturer who is trying to produce their rims 5 cents cheaper than their neighbor down the street. The rim manufacturer does a LOT of testing on these rims including a heat test. It's a standard 4 seconds on, 4 seconds off braking over the course of 12 minutes. The temperature will gradually rise and then plateau at 90 degrees C and stay there for the rest of the test. The rims are rated to 160 degrees C so it's well within that which is why we encourage modulation.
We don't like when products fail and we still realize that ANY carbon clinchers can have a potential for heat problems. I compare this to driving in the snow. If you live in Lake Tahoe in December maybe a Ferrari is not the best car for you. But if you know how to drive in the snow and when it may not be the best idea you can still drive in some pretty snowy conditions (that's the Buffalo in me coming out).
There has not yet been a Zipp firecrest meltdown.
Yes, there has. Granted Zipp has done some great things to advance the carbon clincher market and the products they put out are great (I will never discount that). I do get emails all the time though about "This happened to my **** when I was riding". They have done a lot of work to ensure that their rims can handle a lot and they have the marketing budget to spread that word, but there is still potential for them to overheat when going down a steep enough and long enough hill.