I just meant a Decal modifier (in addition to Decal object) that will turn any custom mesh into a decal instead of basic box/plane, which will be projected onto other objects along the normals of this custom mesh, and not a modifier on the base object, this gives the advantage of allowing you to use custom UV coordinates for the decal and increase the number of possible use cases.
Unlike layered material, this will allow you not to use the second channel of UV coordinates, quickly change the position of the decal in the viewport, and ofcourse apply it to several meshes at once
PS check the attachment
Oh, i see what you mean. Yeah, i agree, that's an interesting idea, but you have to keep in mind that we'd need to deal with projection cage here. In your specific example, it is esentially still a planar projection, but with more complicated meshes you may easily end up with unexpected results, where projection would be stretching, or projecting onto itself. Such decals would be nowhere near as fail-proof as they are now and we may end up with situation, where support have to deal with bunch of false bug reports, "my UVs are perfect, so why my decals are fucked up?" - not everyone knows the difference between UVs and projection, as you can see from this topic alone. Look, i don't want to speak too much against your proposal, i already have been accused in advocating for Corona team too much, i think it's an interesting idea and it's worth to be considered, but it might be not as simple as it seems from a first look. Anyway, it's for the team to decide if it's worth the hassle.
BTW, if you go as far as detaching polygons from target mesh, you already have a decal, you just need to add push modifier to it with small positive value to avoid overlapping, assign decal material with opacity, maybe throw in Corona ray switch material to disable shadowing and you have perfect decal. This technique is nothing new, it has been used for ages in CGI. It might be even slightly faster to render than CoronaDecal, because there's no need for layered material anymore.