here’s a thought:
Star Citizen used mesh decals
UE4’s decal system uses screen-space DBuffer decals.
See where I’m going here?
Star Citizen’s method is really quite old (Hell: the original Homeworld game from 1999 used it), and its super simple:
-start off with a texture filled with all the different bits of detail you want. A ‘warning’ sign, some dirt and scuff marks, bullet holes, etc…
-Make a basic shape. We’ll go with a flat 4-sided polygon here, and UV map it so that the UVs line up with the part of the texture you want it to show.
-Put that polygon in a folder on your hard drive filled with these ‘mesh decals’.
This way, when you’re modeling something like a spaceship, you can import the decals you want from your decals folder and put it on your ship as part of the mesh. This lets you achieve really good detail without any more draw calls or too much overhead or development time on detail texturing.
You’ll see if you go back and have a look at the Star Citizen stuff that this is what they’re doing.
Calling them decals is a bit misleading to anyone familiar with UE4, but it’s more of a general principle.
I like to think of them more like stickers.
So yes, UE4’s built-in decal system does not support metalcy.
But that’s not the type of decal you should be using if you’re wanting to go after that type of decal.