I might have misunderstood what you guys are referring too when it comes to Geometry decals. There is a new Geo-decals feature that made it into 4.9, but it’s experimental niVidia tech. Worth having a look at it just to see if we’re on the same page.
I’m gonna shoot Ryan a PM and see if he has any initial ideas (no guarantees he’ll respond) but in practice I don’t think it’s anything more than reversing the projection effect and relatively easy to do in the Shader. The difficulty I can see is passing the relevant info to the Shader so it can figure out what it’s projecting onto, but World Normal and possibly scene depth could be used to figure it out. Failing that, the new Pixel Offset might have some use too.