ok I think I figured out what you’re doing
I used WorldPositionBehindTranslucency instead of WorldPos as my mesh decal UVs, then disabled TangentSpaceNormal. this is indeed projecting to the world behind.
I actually wish I knew this kind of usage before. it never occurred to me to use it like this, the documentation doesn’t mention it anywhere and I never saw it described here in the forums 
anyway this adds yet another option that I can experiment with 