It don't think it would be busted. Remember, the upgrade is not free. The power there is purchased as an opportunity cost of something else. Unless we get to full item slots. But at that point assassins are hardly a problem.
It makes no sense to not provide an AD caster option that is not black cleaver as they will probably end up choosing another one of Ornn's super items down the line. Most likely Cleaver. Or maybe IE.
MAYBE it is they don't want Ornn and only Ornn to open the door to a lethality first purchase item ADC again. But have to remember it is still a larger gold investment. If Ornn himself did enable that I don't think it would necessarily be unbalance-able.
Not that we even have a clue how much of an upgrade these 1000g upgrades are yet. But I'm getting the feeling they won't necessarily be must rushes.
It is not a good reason in my opinion to directly exclude a class of weapons from his wares. Ornn's teammates can only choose 1 special upgrade and it costs 1000g for the non support stuff. That's plenty of control...
Really I think the lack of an AD caster item is just a symptom of Riot's apathy toward the low elo terrors.
My other guess is that none of these 1000g upgrades are gonna pop out until 6 items by that point it makes less sense to support a flat lethality item. But that'd be a somewhat weak implementation of his passive imo if all the upgrades were just overpriced stat boosts for late game.
EDIT: Wow that's exactly what they are: not really worth until 6 items + elixir and maybe one item swap. zzzzzzz. Doesn't even exist most games and even then completely lacking as a decision point. I was hoping for amped up passives to make each upgrade unique and possibly contend earlier.... Stat ups that late hardly mean anything. Maybe in the future...