Algeria
I've done it....... but on an XR400, not a GS! It's a fantastic route, and I would love to do it again. There were a few VERY difficult sections... tricky even on a light xr400. Some may have been bypassable, and some definately wouldn't be. There's one section of dried out riverbed about 2 miles wide, and it was hell... I personally wouldn't be able to wrestle my GS through it. Most of the route however, is stony desert trail, and perfect GS country. I'm sure many GS's have done the whole route. We camped in the dunes.... it was fantastic.
As I understand it. the Algeria/Morocco border in that area is in permanent dispute, so both countries would claim that you were on their bit! I don't think it's much of a problem... You don't see many people anyway.
The final part of the route, when the massive (600 foot) cathedral dunes come into view near Merzouga is brilliant.
I would say it's definately do-able, and may yet have a go, but I'd want to be in a group of 3 or 4 at least so there was plenty of muscle when needed to man-handle bogged down GS's
Will