Maybe it doesn't work, but I spent an hour trying all kinds of different things trying to get access directly to root as I can do with putty...
When you press [Fetch] to get the 'Public host key', the system copies the content of /etc/ssh/ssh_host_ecdsa_key.pub. So, I tried copying the contents of /etc/ssh/ssh_host_ecdsa_key into the 'Private ssh key' field. Didn't give me access.
I tried to add the contents of ssh_host_ecdsa_key.pub to 'Authorized keys for root', but that content was rejected. I changed the format to just start with ssh-rsa like the rest of the public keys in the box and that didn't work even when I also tried changing the 'Public host key' in the HTML5 VPN definition to the same as in 'Authorized keys for root'. All I got was 'Enter passphrase for key '/tmp/user5902/.ssh/id_rsa:' and that's not the same as the password for root nor any other thing I could think of. Nothing worked.
I put my public and private ssh keys into the HTML5 definition. No joy.
I also tried all of these things with the Private keys in ASCII armor, and, with my private key, the entire contents of the ppk file. Nix!
When I use the password method with loginuser, I have no problem. If this is the only way that works, then it should be documented, but I suspect that the real problem is the lack of any mention of the Private key in the documentation...
Cheers - Bob