Do you know a software that can share a file being downloaded from torrent over http? If the file is beeing seeked by http client, the program start downloading it from requested place. You can see AceStream for example, but it's windows only and works only inside their modded VLC.
Found it! Peerflix. Can download a torrent file or a magnet link. Provides a http link which can be used to play it using any player that can play HTTP links, including hardware players. Tested it a little - playing and seeking works well.
Disadvantage - it uses Node.JS, so it can be problematic to run it on a NAS with 256 RAM (Task manager under windows shows 130 MB RAM and 160 Mb virtual memory).