This was the approach that Transmission used, although it gets kind of weird at times. (The GTK and Qt interfaces can be used with the server/CLI bit built-in,

This was the approach that Transmission used, although it gets kind of weird at times. (The GTK and Qt interfaces can be used with the server/CLI bit built-in, or you can attach them over an HTTP/JSON RPC to a headless server; there is a CLI command (distinct from the server) that runs standalone, and there is a different CLI command that connects to a server over the HTTP/JSON RPC and controls it; some subset of the clients and the headless server come with a web interface that manipulates the "server" bits using HTTP/JSON etc.) Assuming that you solve the CLI bits, using Qt is probably reasonable enough for all your GUI users that aren't running pure GNOME/GTK systems -- at one point in the past, Qt could even make itself look like the current GTK theme (although I'm not sure if that's still true now).