the difficulty is that the wall object comes as one complete object. The layers are apparently only defined by the IfcMaterialLayer. Translating this into the actual object would mean complex boolean operations, that will certainly do the wrong thing in many cases (we have not enough information for how and where to "cut" into the wall). I think there is in IFC a way to represent multiplayer walls as different geometrical objects, but I can't find it back...
We could invent a new type of arch material, a kind of "compound" material, that could host several others with additional properties to define where each sub-material is used. I could see many immediate uses for this, such as multi-layered walls, multi-material windows (or any other kind of "compound" object), concrete objects with a coat of painting, etc. We could define a way to relate these materials to subelements (faces, solids, etc) of objects. But in this specific case, that won't work.
Maybe what we need first s a kind of "layering" tool that would split a wall (or any other object) into layers. A mille-feuille tool