Perhaps this solution isn't what you want, although I Hope you will at least consider it.
The requirement of having a before/after in the Library Module requires you, the human user, to expend a certain amount of effort ON EVERY PHOTO to duplicate a feature that exists in the Develop Module. So, to me, the easiest solution is to use the Develop Module for this pupose. In the develop module, you can toggle back and forth using the backslash key between unedited and edited versions of your photo. It really is that simple.
You will need to create another physical copy PSD not a virtual copy. That is the only way to preserve layers. But beware of doing further adjustments in Develop to layered images. As soon as you choose Edit In >> Photoshop >> Copy with LR adjustments you will be creating a flattened export with the combined LR & Photoshop adjustments.
Missing detail is the before and after would be in the Clients online gallery so they can see the before and after.
What I wish LR had was a option to select and duplicate so it creates a copy with all the layers as well as all the LR edits and keeps this duplicate in that collection. Was hoping there might be a simpler solution.