If you use rtorrent, check out Transdroid, it's a pretty nice Android frontend. The main advantage is that you can simply open a magnet link with it, and it'll automatically push it to rtorrent, instead of having to copy-paste between the app and your SSH client.
I tried to set up Transdroid on multiple occasions, both with commercial seedboxes and ones that I've set up my self - to no avail every time. Right now I use instead Flud on my phone, Transmission on my laptop, and rtorrent on a DO droplet for when I want to quickly download something (and then stream it over ssh + cat + mplayer).
Have you followed the tTorrent RPC setup [1]? As long as you have that working, it's just a matter of configuring the hostname/port and authentication in Transdroid.
I install rtorrent on the droplet, and open the torrent (which I scp onto the box) or magnet.
apt-get update
apt-get install rtorrent
rtorrent file.torrent // use ctr + q to quit
Then I
ssh user@host cat movie.mkv | mplayer -
on my laptop. I use an existing droplet most of the time, however you could easily set up a new droplet each time with tugboat for example - you don't need to install anything extra or do any more setup after rtorrent for this to work. Although you should use .rtorrent.rc if you're going to be doing any serious torrenting, you can just get away with running rtorrent in screen if you want to seed for a bit.
This works fine for most usage cases, however you can't go forwards/backwards in my experience, and the stream will fail if you experience network issues - you'll need to download the whole thing from your droplet to your local computer if that happens.
How do you benefit from this over using a seedbox from a dedicated seedbox host? Is it just because you use droplets that you've already spun up and are using for other things?
The cheapest seedbox I've used that was usable was 10$ per month. You're definitely paying for the support and the time they put into making sure everything runs smoothly. With DO I already have droplets that are running well under capacity most time regardless, so there isn't any added cost. Also, if you want to just download a torrent, you can launch a droplet and delete it as soon as you're done - which will cost a couple of cents at most.
For bettering the mplayer experiences (but quite a bit more involved setup), perhaps one could use a web server like nginx which can server byte range requests. Better to also configure it to only do HTTPS, with some self-signed certificate.
I haven't tested it, but I believe it should work.
