I tried to create primvars:variable on GeomSubset to try to override material parameters, but it doesn't seem to work
I'd assume that the primvars would be composited over the mesh primvars but if ot it feels a bit contraintuitive, is this a limitation of USD?
I'd like to avoid having to assign materials to GeomSubset primitives directly or having to vary the primvars:variable values on the mesh to accomodate for this

This is a limitation of USD.

Perhaps a good RFE here would be a LOP that splits one mesh with geometry subsets into a bucnh of separate meshes? I suspect there would be edge cases that make this coplicated in the general case, but I suspect one could create an HDA that does this with a SOP modify in a way that covered the vast majority of situations…

rather than splitting the mesh I'd probably go with defining the primvars values in SOPs, because split geo will probably not smooth across seams when rendered as subd

I was just hoping that this will work as it seemed logical, now GeomSubset primitives seem a bit ambiguous

I think the good RFE would be with pixar to make it work (unless they have specific reason not to) rather than trying to find workarounds

To me the question is, what was the original intention of geometry subsets to begin with if they can't be used in the way as in tamtes hip. I remember a couple months ago I had a similar issue(different type of set up) with geometry subsets of using groups that couldn't get working.

Fortunately jsmack made the suggestion of using the name the attribute and ever since I've been ‘breaking up’ my geo in sops before bringing them into Lops for specific shading application.