It's good to have your own Guru at work
The following is the solution for anyone wanting to do this. No credits to me, all to the anonymous Guru!
Add your key to avoid being prompted by a password
Find out what socket you are using
serverA $ echo $SSH_AUTH_SOCK
Become root and define your user socket as the one for root
serverA $ sudo su -
serverA # export SSH_AUTH_SOCK="/tmp/ssh-XHGFQ10086/agent.10086"
Now when you ssh from root as your user, you can do so without password
serverA # ssh mysuer@serverB
Last login: Thu Apr 26 14:29:53 2012 from 10.100.1.10
serverB:~ $ logout
Connection to serverB closed.
Now, using our socket again, we initiate an rsync as root using the rsync-path to tell serverB it will also be running rsync as root although we connect as our user.
serverA # rsync -avn --rsync-path="sudo rsync" mysuer@serverB:/root/ /tmp/
receiving incremental file list
sent 139 bytes received 1062 bytes 2402.00 bytes/sec
total size is 69933054 speedup is 58229.02 (DRY RUN)
Voila! We managed to tell rsync to sync the /root folder from serverB to serverA without having to ssh as root. Cooool.