Greetings, developer. I see you have googled far and wide. If you happen to have a similar query as to the one that I posted, allow me to illuminate for you how I fixed it, that your pain may be lessened.
Can you blend phys mats? Yes. Well, sorta. The thing you are looking for is called a Physical Material Mask.
Now if you go search for that, you will discover that there is in fact just about no documentation for that except for some jank page for UE3. This forum link is the best documentation you’re going to get.
In the end, while I was trying to figure out how to get this working, I made a simple realization: In my use case I don’t actually need to figure out which of several phys mats were on a mesh: My needs are more binary. I just put a second invisible copy of the mesh on top of the first one and used a raycast + UV unwrap to figure out if the pixel I was over was one that needed a physical effect or not.
Sorry if this doesn’t help your use case, traveler, but it was the best I could find and I help it was at least somewhat helpful. I wish thee well.