Can Retreat be cast on a champion that was played from the chosen champion zone (rather than from hand)?

Ruling: Yes, you can cast Retreat on a champion that was played from the chosen champion zone. Once a champion is played from the champion zone onto the board, it becomes a valid target for cards like Retreat that return units to hand.

Nuances:

• Cards cannot target the champion zone itself unless they specifically call it out
• The chosen champion zone is effectively an extension of your hand that's safe from hand-related effects
• Tokens can be "returned" to hand (though they stop existing when they change zones)
• You can hide champions like Teemo in the chosen champion zone to protect them from hand-related effects