Any ideas on the temperatures you're expecting? Hot 'n' dry, if you're acclimatised, isn't so awful but dehydration sets in so much quicker when it's hot and you don't notice it if the sweat evaporates. That'll finish them off faster than anything. I've been with two people (one in the Sahara, the other in the Mojave) who just keeled over and collapsed with dehydration - and that was without them exercising. I only just avoided hospital earlier back in June on a hot day (for the UK!) 25 mile run when I got my hydration wrong, and I gave up on a 30 mile training run in August when the shop at mile 22 I was expecting to be able to buy water from was shut.

I'd guess 100 miles over 4 days equals about a marathon per day. Add in the sand factor and they're going to be working hard. Personally, the desert experience I have + 15yrs of running experience would make me wonder whether the 500L of water you mention is enough for 11 people in the desert for a week, particularly given what half of them are doing. I suppose with three trucks as support taking more wouldn't be a problem though.

I've only got a half marathon to do in March so an easy time. This year's one had the opposite problem - too much water, as it was run in a storm. I did the first half wearing a bin bag with holes cut for my arms and head to try and keep warm. Keep us posted when you do it, I'd like to see how things went.