This is definitely being made out to be more complex than it is. Those kinds of details will be generated as part of the entire texture for the object, not as extra geometry or decals. You can do this sort of thing easily indies the texture, using curvature maps and the like. This is really common practice, Quixel, Substance, xNormal and other software all have the ability to generate curvature maps for you to use as a basis.
The post that BrickTop (great name btw if I get the right reference) linked is also not too difficult to do inside the Shader but would require some real thought. You’re basically trying to reverse the decals projection, which is no different really to shadow projection, an inverse ray intersection should allow you to distort the UV’s of the decal and get it aligned to a surface correctly, providing you can somehow feed it the data of what it’s projecting on to, which is the trickier part. You might be able to determine what you need from the World Normal post-process input (if decals can get to it).
Someone get in this thread, he’ll know what to do 
Geometry decals are expensive as hell, they exist in a limited form in the engine but are DX12 features only. Check the blog post regarding UE 4.9’s release, they’re covered briefly there.