Double Decal Trim Blender Unreal Engine Addon SN (Solum Night)
polycounter lvl 5
I'm not good at writing addons, but I made a little addon that works with decals. It still needs to be refined to make it work perfectly, but I don't have time for that right now. If you want you can add or correct something, and enter yourself as a co-author 
https://www.youtube.com/watch?v=dcwq-voZK3I https://github.com/solumthenight/SN
https://www.youtube.com/watch?v=dcwq-voZK3I https://github.com/solumthenight/SN
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It does seem to break materials each time though and reset the viewport camera each time though. Not a big deal of course as the material can be re-applied after the fact. Also, not a huge fan of how it hides the original models. It probably sounds good in theory (and addons doing similar things with booleans tend to do that too) but in practice it's actually quite annoying.
@kanga : not sure what you mean by this ... Yeah, Decal Machine exists ...
The tool of the OP actually provides something that DM actually doesn't have, which is the ability to decal-project arbitrary models without having to go through the tedious step of adding it to libraries. that's actually pretty damn useful for a lot of things ...