Saw this topic on the site, very interesting. Knew it had to be you Luc (not that many people with crazy (but good!) ideas like this).
As I told you before it is hard but certainly not impossible. The bus that made in through in 2000 was a city bus totally rebuild in an enormous camper, this off course made in a lot heavier. It had the help of two trucks (fellow travellers whom they met along the way). I think at least one truck is essential. The border from the Mauritanian side up to Nouadhibou is difficult, but it really depends on which route you take, last December we did it with a 2 wd and up to Nouadhibou no problems at all, rocky underground with very little soft sand a German 207 Merc who was with us got stuck one time but this had more to do with the driver then with the underground. Continuing to Nouakchott a guide is very important! Make sure you get an experienced guide. Maloum (don’t know his last name, will look in my travel diary for this) at that time (2000) did a very good job. As for the beach, don’t drive all the way to Nouakchott, but take the truck route, which is very corrugated but it is your only option. It is difficult to get of the beach, definitely need a truck here (if you’re interested I can give you the waypoint later on) but ones on the truck piste, just take your time and you’re home free. It all just depends on how crazy you are and how much time you have.

Good luck with your decision,
Vincent