Hi mira67 , yes I got it going and it was useful for a time.
The instructions from Aaron ( ryecoaaron) helped to point me in the right direction, and from there I did quite a lot of trial and error to make the system work well. Making the changes to add IOMMU to GRUB was vital and disabling the default NVIDIA and Neveau drivers important, but I also found a useful extra step which took a little bit of experimenting. On the same LAN, (my home network) I installed a Ubuntu VM on my iMac, installed VMM (Virtual Machine Manager) on that, and using that I was remotely able to log into my OMV server and control the assigning of PCI express and USB resources on the OMV machine. It was easier and more reliable to make the changes that way than I was able to manage by editing the KVM config file through the OMV's own KVM management screens.
I tried a bunch of combinations with different virtualized Linux distros (Ubuntu, Pop_OS!, elementary OS etc), and they were good, but not exactly how I wanted to work. I also tried Proxmox with OMV on top - and I also tried OMV with Proxmox kernel and am currently settled on OMV on bare metal with ZFS using the standard Linux kernels and no VMs utilizing the GPU passthrough on the server. Instead I decided to use a separate raspberry pi 4 with a screen and keyboard to play audio and browse and operate as a terminal.
For the "passthrough solution", it ended up that my biggest difficulty wasn't passing the video through to the NVIDIA graphics card, but it was the audio. I was using a USB sound card (Behringer UCA 222) and no matter what sound engine (pulse or pipewire) I used on the VM, the sound passing from the guest through the host was really fickle and completely messed up after distro updates (after I had extensively tweaked bitrates and the like) - so I gave up. It seems that there's no way to prioritize the sending of the audio packets over the USB, so it becomes very broken and distorted as other things assume priority.
Given the nature of OMV being based on Debian, I almost came to the conclusion that a simpler method (and better for the kind of solution I was trying to build) would have been to install a Debian based desktop distro with a good hypervisor kernel and run OMV as a virtualized system on that. Even better would be if the OMV functions could be installed directly into the Debian or Ubuntu desktop as an "OMV server app"... but that would open another "Pandora's box of worms" altogether. 
