I found this video on YouTube that appears to describe how you can do it (I have not used this approach myself).
In summary, you create a new camera for a specific layer (not all layers), then use the unity recorder to export using a targeted camera with a PNG image sequence. The PNG files have transparency, and you use a 3rd party tool to create a video file from that image sequence (pick a video file format that supports transparency!!). The video uses Photoshop, but I have done it with Adobe Media Encoder or Premier Pro directly (when experimenting with video quality without transparency).
Thanks heaps! I’ll try it! I had a mega tight deadline so I had to think on the fly. What I did was make a new volume, loaded the HDRI slot with a completely black HDRI I made, used a little contrast in post-pro, then inverted this black and white Recorder movie in Fusion. Long-winded but this method above will be way better!