During a showdown at one battlefield, can you play hidden cards or actions that target units at a different battlefield?

Ruling: Yes, you can play hidden cards and actions at any battlefield during a showdown, even if the showdown is happening at a different battlefield. The only restrictions are what the cards themselves specify.

Nuances:

• Hidden cards still have their normal targeting requirements tied to their battlefield
• There are no "only while there is a showdown here" restrictions on card play