I might be wrong, but I don’t believe this is possible:
Whenever you’re working on a set of materials for a mesh that has more than one, the preview mesh in the editor always puts which material you’re working on at element 0 (the first material slot). All subsequent materials you create will also preview on that same part of the mesh and there’s no way to preview what material you’re working on in the correct material ID of the mesh itself.
Example: A character has a backpack that is a 2nd material. Building the material for the character is pretty easy because that would be material element 0, so all your edits show up on his body. But once you’re creating the backpack, the backpack part of the model stays blank while the material you’re building for it is previewing all over his body.
Down in the preview mesh options, it would be nice to be able to assign which element # we’re working on so we can preview the current material on the desired part of the mesh.