@Cino:
Emarl would be the guru if its possible. Thinking a code change would be needed to allow a feature like this. Do you have access to all the clients running bit-torrent software? You could set static ports then create an alias to direct all that traffic thru the gw you want. Thats what I did for my network
Well that's what I'm doing now. I basically put the before-last-rule to be that ALL traffic of the bittorrent machine (192.168.0.10) be NATed to the DSL connection. Above that rule, I put that port 80,80,443 (and a few other ports) from 192.168.0.10 be sent to the cable connection.
So far it works ok, but the problem is with the trackers running on HTTP will be contacted by my cable connection. So getting incoming connections on my DSL for bittorrent is a bit slow, as the DHT and peer-sharing functions need to kick in for my DSL connection to be known to the other peers.
but it works none the less and maybe I'll leave it like that since I don't want to take the chance that L7 layer filtering (if I'd get it to work) would fail one day and reship everything to the cable connection, costing me a pretty penny in overages.