To create a Bittorrent magnet link, use rhash :
rhash --magnet --bt-private --btih --bt-announce http://tracker.example.com:6969/announce yourfile.mkv
The --bt-private option is like the ctorrent's '-p' ("private") option which disables the use of DHT, PEX or other trackerless peer-exchange methods by other clients when sharing this torrent - if you use your own tracker, the swarm will be circumscribed there.
Then copy yourfile.mkv to rtorrent's 'incoming' directory (typically ~/bittorrent/incoming) and start seeding using the magnet link handler found here. Using a rtorrent setup dedicated to seeding, you might also consider making ~/bittorrent/incoming a symlink to your file repository - I'm thinking about that for sharing a whole collection (but that will require a bit more scripting to avoid recalculating all hashes each time the shares are updated).
This recipe is untested... I had a similar question and took this opportunity to research answers - tell us if it works !