This would break champ select because if you veto before the pick, it's not very useful (because they could still change their hover last minute and lock in before you have a chance to veto), and if you allow veto after the pick is locked in, the enemy may very well already have been planning their team comp around the locked in pick, thus you're screwing the enemy over. Furthermore, allowing veto after pick would enable a team to simply veto any champ select that they get outdone by the enemy in (ex: the enemy picks a counter to your lane) - which isn't really helpful.
I can't think of a way these can be reconciled.