libreCMC uses dropbear SSH server. I was a little confused because I tried to put my id_rsa.pub and other *.pub files into ~/.ssh/authorized_keys, but that didn’t work. I discovered that you can use your ssh public keys, but you have to put them into /etc/dropbear/authorized_keys instead. Also, be sure that the authorized_keys files is set to 600 or 640 permissions.
There is also a box in the LuCi interface under System >> Administration for pasting in SSH keys. A trick though is that you must make sure that the paste process does not add any line breaks. And that is something that is pretty easy to do when trying to copy and paste out of a terminal.
I’m thinking one could hack the init files or something to make it use ~/.ssh/authorized_keys instead. But I believe that the root home directory is not preserved across firmware upgrades, so this would not be a good idea.