Media Capture Alpha and Reflection

Hello everyone.

I try to do augmented reality with Unreal. I use media capure with AJA card to output fill and key video signals.
Everything works fine but I can’t manage key signal with reflections in output.
I try to do thing kind of thing : Digital Characters - Injustice 2 on Vimeo

I would like to have in the key signal the refletions of the mesh with alpha control. A kind of a shadow catcher but for reflections.

Thank you.