I've just read a few posts from a while ago that the shaper only looks
at a packet once, in the case of outbound that would be OPT1.

It would be useful if the shaper wizard mentioned this aspect in big
bold letters, i.e. if(count($interfaces) > 2 || $filteringBridge ==
true) { print "<b>Please note</b>: if in bridged mode, you will have
to manually change rules for outgoing trafic to specify the internal
bridged interface and the "out" direction." }

I've never edited m0n0 source before, but I can have a go at adding
something like this if others are busy.