Yes it would,because the attack is still doing the damage and Focus Band states that it is activated if an attack KO`s the Pokemon it`s attached to.Here was the last ruling:
Q. Would an attack that "places damage counters" trigger Focus Band if the defender was KO'ed from it?
A. Yes, because it doesn't matter how the damage counters landed on it; if it was an attack and it would be KO'ed, then you check the band. (Sep 13, 2001 WotC Chat, Q118)
`Sensei