Can you play Zhonya's Hourglass to base (hidden) and then save a character in a battlefield?

Ruling: Yes, you can play Zhonya's Hourglass hidden to base, but you must reveal it (at reaction speed) before it can save anything. It will save the next unit that dies after being revealed.

Sequence:

• Play Zhonya's Hourglass hidden to base
• Reveal it at reaction speed before a unit dies (you cannot wait until a unit is already dying)
• Once revealed and in play, it will save the next friendly unit that dies

Nuances:

• You never choose which unit to save unless multiple units die simultaneously, in which case you choose which one to save
• Zhonya's must be revealed before it can affect anything - you cannot reveal it after damage has been calculated and a unit is dying
• The card must be in play (revealed) for its ability to work
• There are pending clarifications about whether Zhonya's only affects units at the battlefield where it was hidden