That's close but I want arbitrary BT files, not a hand curated content selection.

You can use peerflix[1] for that, eg:

    peerflix "magnet:?xt=urn:btih:44052304ac07aa461b4828c63c52a44f34df2ce7&dn=FIFA.World.Cup.2014.Group.B.Spain.vs.Netherlands.720p.HDTV.x264-&tr=udp%3A%2F%2Ftracker.openbittorrent.com%3A80&tr=udp%3A%2F%2Ftracker.publicbt.com%3A80&tr=udp%3A%2F%2Ftracker.istole.it%3A6969&tr=udp%3A%2F%2Fopen.demonii.com%3A1337"

And then watch at http://localhost:8888 with your favorite player.

[1] https://github.com/mafintosh/peerflix

In PopcornTime, you can drag and drop arbitrary BT files.

This doesn't seem to work in the latest version.

If you're referring to Time 4 Popcorn, that's not it. It's also malware.

There are now several forks with different features.