there is no option like this but you can do it by some kind of compositing in a postprocess material.
Use
- your render target
- scenetexture:custom depth (black & white - actor rendered to custom depth should be black- multiply scenetexture:custon depth by something like 0.0001).
- a vectorparameter, simple rgb color, black.
Then use custom depth render as a mask:
- to display parts of the render target where custom depth render is black
- to display a black background when custom depth is white.
So you can isolate the actors you want.
Background won’t be transparent but you can then further add the background you want, using this same kind of compositing stuff in the same PPmaterial.
Hope it helps.