Increasing his bard level for the purposes of bardic music unlocks new bardic music abilities - the equivalent of gaining new levels of spells. That would be a huge advantage, even if it was just some additional effective levels. I don't see any way a feat could allow bardic level = HD.
I think it would be fair to allow a feat to gain one bardic music ability, with a prerequisite of bard level 2 less than required to gain the ability. I don't know if that feat exists (not in Complete Arcane or Complete Adventurer).