I have yet to confirm this fully but I believe that the OpenMediaVault Rsync server configuration will not support sending user and group information to other clients because of the 'use chroot = yes' parameter in the rsyncd.conf file.
When running rsync on another machine to pull data off an OpenMediaVault and you want to preserve user and group information (or use the Archive option), it might be worth adding 'use chroot = no' to the OpenMediaVault Rsync -> Modules -> Extra options.
See the http://www.samba.org/ftp/rsync/rsyncd.conf.html use chroot section for more information.