A client supporting Transparent BitTorrent adds the application/x-bittorrent MIME type to the Accept header field of any HTTP request, or as indicated by user preferences.
In response to such an request, a server supporting Transparent BitTorrent evaluates the feasibility of a BitTorrent download, for instance based on the file size (as BitTorrent don't make sense for smaller files!).
If BitTorrent transfer is feasible, the server delivers the torrent metainfo data (appropriately labeled with an application/x-bittorrent MIME type). If BitTorrent transfer is not feasible, or the BitTorrent MIME type was not present in the client request, the client is served the regular file.
Notes:
To use Accept-Encoding would be about as logical. I'm happy to hear your opinions.