Since we are talking libtorrent: To be honest the best and only sound option here would be to port rtorrent to harmattan. The project is very well documented and the whole source is available. That's pretty much the most decent client out there (and it can be extremely efficient) which is surprisingly easy to handle from command line and very scripting/plugin friendly.

There's really no use to develop or rewrite a 'native' torrent client from meamo. No private tracker will white list it anyway. And transmission? Transmission tend to be inefficient and too often in the past its version caused problems.