OpenSSH 8.0 released

You could try what excors wrote about using sudo-rsync with a password.

Another thing you can try if your source machine allows root-login-via-ssh is reversing the direction of the transfer: instead of running rsync on "source" ssh'ing to "destination", ssh from "source" to "destination" with a reverse port forwarding to "source:22" and run rsync on "destination" with "localhost:<forwardedPort>" as the source address.