I've just found that thrown melee weapons use the energy cost, damage, and to hit bonuses from the skill. A thrown whip works as well as wielded! So kinda perfect for a duelist in a pinch.
But it gets better! Thrown axes and hammers work this way, I havnt tested thoroughly yet, but it seems to use the best score- best to hit or damage or energy cost from the 2 skills instead of adding together.
It also works this way with swords and 2 handed weapons.
Spears work this way too.
Wielding a bow such as sun's messenger works this way, you won't gain weapon marks, but it's an undead slayer still and you can take full advantage of the great damage it can cause from both the skill and the +12.