One way iproute could be helpful here by creating a /run/netns/mnt, onto which one 'iproute' mounts namespace was bind-mounted. Then 'ip netns exec' could setns into that mount namespace, *then* unshare mntns. The /run/netns could be a slave to the host but peer with all its child namepace. (I guess it would have to be /run/netns_mnt for that to be sane)
One way iproute could be helpful here by creating a /run/netns/mnt, onto which one 'iproute' mounts namespace was bind-mounted. Then 'ip netns exec' could setns into that mount namespace, *then* unshare mntns. The /run/netns could be a slave to the host but peer with all its child namepace. (I guess it would have to be /run/netns_mnt for that to be sane)