Zoneminder supports hardware acceleration in the new version. I have been able to add the '-hwaccel auto' parameter to the ffmpeg options and I've not have any problems. When I try to add the vaapi hardware acceleration to the cameras Unraid locks up and has to be hard reset. When I use the 'ffmpeg -hwaccels' command in the docker, it says vaapi is available as a device to use for acceleration.
I have an Intel GPU in my server so I believe qemu and libvirt are picking that up.
For the time being, I would recommend caution with the hardware acceleration feature on the cameras. I have read that the vaapi driver has had issues with later Linux kernels, but those posts are a bit old.
Just a heads up.