Oh, btw, I don’t know when this was added in but it appears that since at least .25 each material has an array with up to 8 physical material maps.
This is a screenshot of the material.
It probably works similar to what I described.
The mask and the default material can be set in the Physical Material section. I assume if you setup a mask you can then paint the areas into it using HEX color codes to get the possible base value mixes (FF0000 - red, 00FF00 - green, so on).
Pretty dope to find exactly what I pictured already implemented 