Sure, it'll add some aroma and a bit of flavor.
But I wouldn't start carbing just yet. When you purge the keg to remove the ball you'll lose a lot of the volatile aroma compunds. Just hit it with enough gas to seal and remove the ball 3-7 days after you added it. 3 days if you want just aroma or 7+ days if you like the vegetatitve green flavor associated with long dry hopping... the latter seems appropriate for an ESB.
Primary: air and sadness =(
Kegged: air and sadness =(
Bottled: English Barleywine (brewed 9/26/09 -- bottled 5/5/10)
LET'S GO LA!
PLAY FOR GLORY, THE GLORY LA!