Okay
I was playing around a bit, and found the solution that best fits my needs:
I created some ZFS datasets on Proxmox, and configured a network bridge (without briding ports - so like a "virtual network", in my case 192.168.1.0/28) between Proxmox and OMV (with VirtIO NIC). Then I created some NFS Shares on Proxmox and connected to them via RemoteMount Plugin in OMV. Speed is like native (the VirtIO Interface did incredible 35Gbs when I tested it with some iperf Benchmarks), and now I dont need and passthrough. Works like a charme for me