This is pretty rad. I ended up wanting to ssh directly into my containers; in order to do this I had to edit /etc/ssh/sshd_config on the macpine box (using alpine ssh lxd.)

AllowTcpForwarding yes # change this from "no" to "yes" in /etc/ssh/sshd_config

After running /etc/init.d/ssh reload, I was able to use the macpine LXD VM as an ssh jumpbox (via ssh's handy -J option.)