A relative is doing this, so I'm taking an interest - doesn't look that hard...
When we were in the Alps July 2015, temps hit 40C, so I can only imagine what it's potentially like out on the tarmac up Galibier and d'Huez. Where is the closed tunnel of which you speak..?
It obviously depends on time of day – during the event, the people doing the faster times will clearly be going up Alpe d’Huez at the hottest time of the day. They allegedly have a sprinkler system in operation in places for the hottest hours on the Alpe but I can’t confirm this as it’s always been long gone by the time I get there (or never existed).
For the slower riders the Telegraph is the worst for heat – I suffered there a lot the second time I rode the Marmotte, as I had a very late start and the water station half way up had run out of water – I had to do the last 6km or so on about two mouthfulls of water.
I can’t speak for the faster riders and how they suffer on Galibier but I’ve never found the heat a problem – I don’t know if it’s the air thinning but I have been up Galibier on a sunny day and not really suffered, or at least not thought I was suffering more because of the heat. My problem on Galibier is usually the cumulative effect of all the climbing over the day – by the time it starts kinking up to the steeper sections I’m usually done in by then.
Despite being one of the Sturdy Chaps (TM Charlotte OTP) I don’t really suffer in the heat as long as I’ve got lots of water – this could well be because I’m not going as fast as some of the other people and taking it more steadily. That said I have seen other people suffering terribly from dehydration and dropping out on this and other events.
The Telegraph was where I noticed the big variance in heart rate going from sunny patch to shady patch.