Hi Pilou - non-planar faces do not have a single perpendicular direction to them, a curved surface has surface normals pointing in a variety of different directions.

So for non-planar surfaces there isn't really a way to extrude along "THE" surface normal because there is not just one.

I suppose for a non-planar shape that has symmetry around a kind of central point could make sense, but unfortunately it's difficult to figure out patterns like that from algorithmic code. Remember - code can't easily detect shape patterns and use judgment like you are able to do.

- Michael