Ah, OK.
Basically what I want to be able to do is just use a material function on any arbitrary channel of a texture. I am texture packing greyscale mask textures, and wanting to do stuff with them. And I might want to use the material function with the R, G, B or A channel, it depends. If it was always consistently just the red channel, then I could easily make the material function extract just the red channel in a way similar to what you’re suggesting.
I think the only option really is to just copy-paste the contents of the material function into whichever materials I want to use it in.