Can you play hidden cards from a battlefield that isn't currently being attacked as a priority reaction?

Ruling: Yes, you can reveal hidden cards from any battlefield at any time you could play a reaction, not just from the battlefield currently being attacked.

Nuances:

• Hidden cards are not restricted to only being played when their specific battlefield is involved in a showdown
• This allows for strategic plays like revealing a hidden champion on one battlefield to trigger effects that benefit units on a different battlefield during combat