Thank you for the positive feedback.
To define a user/password append :/my_user:my_password to /storage/.kodi/addons/service.rutorrent/config/httpd.conf and restart rutorrent with systemctl restart service.rutorrent.
I might add this feature to a next iteration of rutorrent, but it is not one of my priorities.
You can find additional information about BusyBox httpd at BusyBox HTTP Daemon (httpd) [OpenWrt Wiki].
Be aware that any changes to httpd.conf configuration will be lost each time rutorrent is updated.
More importantly, the add-ons were not designed for direct connection to the Internet, especially not rtorrent/rutorrent.