If you’re doing this in a static way (just placed in the world) your best bet is tweaking geometry / textures as needed.
If you’re doing this run-time (say, bullet mark on a wall), then your best bet is to place a static mesh actor with just a quad in the world where you need the decal to be. Casting a few rays can get you the position / normal for the quad. You can even do some edge / corner casts to determine if the decal would be partially “floating” and either modify or just not add it. Decals on curved surfaces won’t work as well with this however but it’s mobile VR so… you need to limit your expectations of what you can / can’t do right now.