On an lxd container one could go this way:

lxc profile create losetup-profile
lxc profile device add losetup-profile loop6 unix-block path=/dev/loop6
lxc profile add mycontainer losetup-profile

Adapt the profile name, loop device and container name to suit your needs.

More useful info at https://www.forshee.me/container-mounts-in-ubuntu-1604/