Forgive me if this is common knowledge, but I couldn't find any forum references to rsync parameters, and was going to report a bug, but I found a workaround for my issue.
You can specify environment variables (such as %RSYNC_USER_NAME%) by adding the appropriate option (even if provided in the web ui) under extra options.
The UI doesn't let you type %RSYNC_USER_NAME% in the User field, nor * in the Group field.
However, I was able to work around that by adding uid and gid directly to extra options, which places them at the end of the module config.
This works as expected, and files are stored on the server as the authenticating user.
Perfect for home directories!
You can find the available variables on the rsync documentation (which happens to be linked to from the OMV UI): https://www.samba.org/ftp/rsync/rsyncd.conf.html