Hi. I'm not sure what you mean by "multisubs within multisubs" but regarding materials, you pretty much describe thebworkflow I've been using for years.
If you make the same materials for different render engines and keep the names the same, then it's possible to replace all materials in the scene within a few seconds, either by loading a matlib file or by getting the materials from a max file. The latter option is the one I'm using more and more since this way instanced maps keeps being instanced.
I've used this for files that I must deliver with native max materials, but I still want to render and work with using 3rd party renderes, or when working with Unity, where I've build the same materials in Unity as I'm using in max.
Hope it helps