As far as I understand, the mp options and lxc.mount.entry are the same except for the /var/lib/lxc/$VMID prefix. Are both variants reflected in the GUI? Is mixing problematic?
Which lines do you change for the mount points to be visible in the GUI. Do have a diff to be applied to the Proxmox code? Do you automatically re-apply it after every update?
We just ran into the limit of 10 mount points per container. Would it be sufficient to adjust $MAX_MOUNT_POINTS on all servers in the cluster? Could this become a per-cluster configuration option?