I’m still not quite sure what your are trying to do, but here are some links that seem relevant.
Cube Render Target sounds the most relevant. It’s like a scene capture, but for cubemap (maybe you’ve done this already)
I found this, which does scene capturing on the fly. I don’t think it’s exactly what you’re after, but may help point you in the right direction.
There is also reflection vector in bp or material editor.
If you could provide more info, I’d be happy to indulge.