The current version of LXD is 2.21. There is a patch that was accepted at https://github.com/lxc/lxd/issues/2504

and should make it in LXD 2.22 (or is it 3.0?).

It adds the functionality of a new device, the proxy device that currently can do TCP forwarding (UDP planned for the future).

It looks like this,

lxc config device add xen1 proxy0 proxy listen=tcp:192.168.50.196:80 connect=tcp:10.0.10.88:80 bind=host

Therefore, once the new version of LXD is released, I plan to write a post about the proxy device. Then add some backup info that UDP and Unix sockets are pending work, and people can use iptables for those cases.