In that case, I think we have some small misunderstanding regarding the meaning of layer as used in bella. A layer in bella is not something for doing decals or such, it is literally a simulation of a thin layer of dielectric over the substrate material.
For the kind of thing you are talking about (I think) we have the blend and stack materials, which do what they sound like. If you want a parallel to something like maxwell, it would be like having an mxm where each layer referred to another mxm file, and you could blend or stack them, mask them, and so forth.
But note, you don’t “embed” materials here, as you would have to with mxm, you just refer to them, and you can therefore refer to one from multiple different blend or stack materials. So you can set up, say, a particular dirt material, refer to it in multiple blend/stack materials, and have all those materials be automatically improved, when you then go back and improve the single dirt material.
So to try and correlate this with your comments above, I infer that your main point/concern is that if you have, say, a stack material that refers to 13 other materials, you are interested in making sure that we have a convenient list UI to show those 13, where it is easy to get an overview, to select individual ones for editing, to manage masking of them, and so forth.