antc
I was able to get this to work using an editmaterialproperties lop. I had less success with an editmaterialnetwork lop.
Attached is a hip file where the network on the right creates a material called "base_material" and exports it to materials.usdnc. The "base_color" parameter of "base_material" is set to a purple color. In the material network it's important to click "Promote Parameter" on the base_color parameter (using the gear icon to the right of the parameter). If you don't promote the parameter it won't show up populating the editmaterialproperties lop (via the "Create Parameters" button).
The network on the left then imports materials.usdnc as a sublayer. The material is assigned again, and then a editmaterialproperties lop is used to make the base_color a greenish color. Everything else describing the material is coming from the materials.usdnc file. If you go back and make a change the the base material on the right, re-export, and then click "Reload Files" on the sublayer_base_material node you should see it update.
You don't need to export the materials to usd for editmaterialproperties to work of course, although if you're working across several assets in separate files it's generally simpler to share a usd file than a hip file imo. But if you wanted to avoid exporting usd files you could put your base material setup in a hda and use that in multiple assets instead.
You are absolute legend!
Following your sample file, I managed to replace color parameter with File parameter and I was able to plug whatever image I needed to that material. Super!
Now my last challenge here is how to alter that material name so I can read in two or more base material and modify them by using specific image for base color and assign them using group using Material linker.
Right now if I add another base material, it will obviously overlap with first one and I cannot apply it to different section of my geometry using subset groups. Is there any way of altering material name just to be able to see it in material linker?