I run the following to get the router to forward connections to Transmission:
iptables -t nat -I PREROUTING 2 -p tcp -d `nvram get wan_ipaddr` -j ACCEPT iptables -t filter -A INPUT -p tcp -j ACCEPT
Whenever I make a change to the QoS settings, these entries in iptables get reset and I have to run them again. Smells like a bug to me, but perhaps there's a checkbox somewhere that I haven't checked?
Note that, for some reason the Transmission port-forwarding isn't working hence my need to do this in iptables in the first place.