With VRCMirror, how can I reflect only the selected layer, without the skybox background or any background?

I’ve baked a box projection reflection probe and put it on a background mesh behind my mirror mesh. I want to reflect only players’ avatars (and maybe one other layer), and have the specular reflection on the different mesh be the background.

But by default, mirror reflection always renders the skybox. Can I avoid that? Alternately, can I make the mirror reflection just show the relfection probe? I’m pretty sure something like this is done in the Summer Solitude world.