So far, I've witnessed one blowout with a rim brake in the alps. However, the cause was not heat but that the rim was braked through (i.e. a section of ≈ 120° split off). (Fortunately, nothing bad happened, and that guy was in a group that had a van alongside with them)
Take home message: take care that the rims are still sufficiently thick.
The one descent so far where we've been taking some extra caution was going down from the Vršič pass at 1611 m into the Soča valley at 780 m (≈ 9 % for a bit more than 9 km). The first part can be done at comparably high speed so air resistance helps. But the lower 5 km (ca. 500 m of the elevation loss) have one hairpin bend after the other (IIRC 19 in total). While there's decent asphalt in between, the hairpins tend to be done in cobble stone and may have loose sand on top. In other words, forget about using drag to get rid of excess energy. Our bikes were loaded with full camping tour gear (back panniers + low riders in front). On that descent we did stop several times to check temperature (and let the lined up cars and buses pass). While I don't have thermometer readings, we did reach "finger says ouch". We also employed water cooling: wet the rim surface - when you are around, the rim is dry again ;-)
(BTW: I don't recommend Vršič as there was a lot of traffic - everything from bikes over motor bikes, cars to fully-grown motor coaches)
You may be interested in these records of maximum rim temperatures together with whether they did have a blow-out or not.
I'm not at all sure that tubeless tires stand rim temperature any better than the tube variety.