Fiddling with the bard, I found a power I like with metafluff I dislike : Jinx Shot.
Fluff: Your expert shot renders your foe unaccountably clumsy.
Crunch: Charisma vs AC. Hit creature falls prone when missing an attack before the end of your next turn.
Keywords are arcane and weapon... no instrument there. The fluff doesn't give much - the power's title is much better here.
Let us look at the crunch to see if we can come up with a better fluff. So, the bard hits with his ranged weapon, using his charisma... and "jinxes" the enemy.
Hitting using charisma means that the bard uses his bard techniques to secure the shot and affect the creature's psyche. Taunting fits well here - enticing the enemy to attack more clumsily in a fit of rage.
To make this different than a normal bluff or intimidate during battle (you know you can do that, right?) and to differentiate wit the other bard powers (''Vicious Mockery comes to mind), we need to make the taunting more in line with the "jinx" idea.
So what could bard do to give bad luck? Really depends on the bard's style. I can easily picture a trickster bard saying something à la "your shoe is untied".
However, a bard is the ultimate performer - it can be in the actions...
Kellen aims his crossbow at the enemy, ready to shoot... The enemy catches the movement of the bard's arm, noting where the bold should land, he readies himself for a dodge. For a split second, the bard pulls his aim, a puzzled look on his face as he stares at the goblin's feet.
The enemy, taken aback by this unexpected pause, looks at his feet. The bolt hits him straight in the arm. He angrily jump toward the bard, swinging madly. In his haste, he steps on one of his boot's loose lacing and lands on all fours.
"Looks like your boot's untied", says Kellen, lifting the edge of his top hat with his free hand.
To my sense, the fluff should render a sense of the bard's action being tied to the result - not his marksmanship.