Make purchasable, but at the end of the event. Looters get first. What about any legend cost minimum 500 haunted, but also drains your entire reserve (so you can't buy them all in a single event)? You can loot a lot and get 500, but still walk away with something. Maybe you lucked out and got Dark Whisper, but would also like the doll. That way you could walk away with something, but also have something to look forward to next year.
For the El Patron/Silver Freeze issue, I think in general those should be removed from bosses that drop something unique. SF or a new legendary knife should be found at Foulberto, as he is one of the most fun bosses to fight -- but often isn't "worth it" to many.