I actually would say leave DW gained only by /nin or /dnc
That said, I'd vote for a new Job Trait.
main-gauche
Allows user to equip a parrying-dagger in their left hand. (sub slot)
Level 40 - RDM (10% Delay reduction)
Main hand must be a sword, this does give it available to all as /RDM however the only real jobs that /RDM either have native DW or can't equip Swords/Daggers
Seeing as we're depicted as a fencer, with main weapons being rapier's/épée and having B skill in both Daggers and Swords, etc it fits the image perfectly, also it allows for the additional damage, the 10% is so they can make it so
Enhances "Dual Wield" Effect equipment works with it, along with a possible new category for equipment
Enhances "main-gauche" effect.
http://en.wikipedia.org/wiki/Parrying_dagger