So, if Ubuntu has a reasonable work around, that gets past the ZFS licensing issues and provides desirable repo features for @ryecoaaron and OMV's Dev's, using Ubuntu LTS as the base seems to make sense.
If we are "just" trying to fix the zfs issue, using the proxmox kernel/repo would solve that. I have added support to install the proxmox kernel/modules to omv-extras and the zfs plugin that should be available in the next few days.
If anyone worries about this combination then they should know that proxmox is a repackaged Ubuntu 18 kernel with a Debian Stretch userland. So, there should be no problems with using the kernel in OMV 4.x.