I did it once. I slept in Valcarlos and in the morning I went up the D128 (in fact, there is also a white-and-yellow marked path that avoids the road, but I found it difficult to follow the markers). The D128 is actually a very similar type of road to D428, the one taken by all the pilgrims on the Napoleon route, except that it is longer, because instead of being mostly straight as D428 it has several "switchbacks", which I didn't mind because as the result, the gradient is gentler. You join the Napoleon Route a bit before the Food Truck.
You might be wondering why anyone would sleep in Valcarlos and then go all the way up towards Thibault Cross from the other side. Why not just follow the original path through the valley from Valcarlos to Puerto de Ibañeta and then down to Roncesvalles? Well, I guess the only answer is: for the views, which are spectacular on the Napoleon Route. Similarly, you might say, why would people do the Hospitales Route on Primitivo, instead of staying in the valley until Pola de Allande and climbing up to Puerto del Palo as late as they can? They probably do it mostly for the same reason, the views on the Hospitales route. Nowadays there are very few "dangers" lurking in the valleys for pilgrims. From a logical point of view, climbing the Lepoeder Pass at 1450 m makes no sense at all, if you can walk through the valley and only ascend to about 1000 m. But people still do it.
As an added bonus, during the whole arduous climb on the D128, you will have the whole Pyrenees to yourself! Nobody is puffing and huffing behind you, as is always the case on the Napoleon Route in the early morning.
