When I was in Muang Sing, 3 years ago, I met 2 people from Holland. They wanted to do that trip. They had Lao Honda Baja with big tanks. Probably the rental company (was it Green Discovery?? I cannot remember) can tell you if they did it or not.
Maybe Jimoi knows about that trip.
I would be interested in the next leg to the north. Is there a way from Xieng Kok, along the Mekong to the chinese border?? Wy don´t you do the whole Mekong?? Even more interesting!
I tried to do the Muang Sing to three countrycorner Burma/China/Laos, but failed....