just think though, if everyone who used bittorrent did what you're trying to do, where would the client be today? not very far. you're trying to limit what's made bittorrent such a unique concept: FORCED uploads... where you can't click "do not share", and just leech off everyone else.
you're not the only one that's got that problem... i have at least 3 computers always on in my house, this one being one of them, and then occasionally one or two more will be plugged in at a time (depending who's got their laptop on)... so do what i do (and what you suggested): only torrent at night, when you can contribute to the bittorrent network, and not worry about sucking everyone elses bandwidth

also, as i said, unless you limit your DOWNLOAD speed too, you could limit your upstream to 10 bytes/sec, and the internet would still be slow for other computers on your network