I was going to add a few more pressure plates to the list of ones that work for the grinder, but it doesn't seem to use a tag for this. It just seems like something that would be configurable by datapacks in some way.
Several tags for different levels of multiplier, maybe?
forge/pressure_plates/wooden
forge/pressure_plates/stone
forge/pressure_plates/metal/basic
forge/pressure_plates/metal/precious