Can hidden cards with abilities (like Blastcon-Fae or Fight or Flight) target creatures on the other battlefield when turned face up or activated?

Ruling: No, hidden cards cannot target creatures on the other battlefield. Hidden cards must target on the battlefield where they were hidden.

Nuances:

• Hiding a card gives it the "Here" restriction
• This applies even if the card text doesn't explicitly specify "this battlefield"
• The intent is to be even stricter, and Hidden will be reworded in the next rules document to clarify this