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What I mean is the editing of the layer mask itself can only be done destructively (afaik, do tell me if I'm wrong). For example, you cant use a smart object as a layer mask, nor apply adjustment layers to them.

I needed to use an actual layer (a smart object) as a mask based on its luminance (the same as a layer mask) so that certain elements of the mask could be edited easily and non destructively without having to go back and forth copying/pasting things into layer masks from other layers which I'd need to keep showing and hiding.

In the end I worked out how to use 'blend if' in 'blending options' to effectively create transparency from luminance in a layer, so that I could then just use it as a regular clipping mask.