errrr, yes and no....
no, there are none that run in a client/server fashion (i spent weeks looking for the same thing). however, as always with linux, a little jiggery-pokery might make it work.
check out rtorrent on
http://libtorrent.rakshasa.no/ its a bittorrent client made to be run on headless boxes and the like, via screen, and it is pretty good. not exactly client/server as you described, but with a little tinkering with screen, it could serve your purposes. its what i use to do basicly the same thing as your suggesting.
if that doesnt suit your needs, and your going to write your own, they have libtorrent on that site too (rtorrent depends on it) that might be usefull to use in your code.