FWIW, one workaround I've found so far is to rely on Create Point, and clicking multiple times in Repeat mode to sketch using dots which do snap well to the surface of objects. It works alright but of course but it very far from the fluidity of freehand sketching :



Also, temporarily changing the CPlane seems to be a decent option too. It really seems like the functionality to sketch on the surface of objects is almost there but not quite ...

Lastly : is there any way to set specific objects so that they do not get picked up by Object snapping ? This would be useful when drawing the final/clean curves on top of the sketched ones.