How to mount LVM in unprivileged LXC container

Hello, I have an LVM on the host containing a BTRFS filesystem that I want to mount in an unprivileged LXC container. I am using regular LXC not LXD or Proxmox or anything else.

lxc.mount.entry = /dev/mapper/ubuntu--vg-that--home var/lib/that btrfs defaults,nodatacow,subvol=that 0 0

(I also tried with an absolute path destination mount)

I attempted to use setfacl to grant rwx permission to the user that owns the unprivileged container:

setfacl -m u:that:rwx /dev/mapper/ubuntu--vg-that--home

It did not work, I still get:

 utils.c: safe_mount: 1221 Operation not permitted - Failed to mount "/dev/mapper/ubuntu--vg-that--home" onto "/usr/lib/x86_64-linux-gnu/lxc/var/lib/that"

I found a Proxmox forum entry that said it was possible but they didn’t explain how to do it outside of ProxMox. Can anyone help?