Hey,
I just wanted to let you know that I have integrated KIO with KTorrent and have also extended the magnet-uri schema into an url schema which allows me to browse files in torrents like http or file ressources would allow. This is pretty flexible. From HTML5-video streamloaded from torrent over browsing a Wikileaks mirror up to using torrent-collections like local folders in all KDE applications works already.
Have a look at the latest three posts here: http://whilos.blogsite.org/ Especially watch the long screencast, to get an idea what you can do. I would love to see that working in Gnome/GIO with Transmission as well and as you seem to be the primary torrent client there you are the candidate for this.
I have done that with talking to KTorrent's extensive DBus interface from the kioslave. That way I only need one daemon (KTorrent) running in the back handling all the details of BitTorrent and have still its nice control ui to check what is going on. I guess something similar could be done with GIO.
It would be helpful to discuss the new schema before it gets used.
Cheers,
whilo