Can Zhonya's Hourglass be played from hidden without a unit dying?

Ruling: Yes. A hidden card gains Reaction timing while facedown and may be played any time a card with Reaction may be played, regardless of whether its replacement effect has a valid event to replace. You can play Zhonya's Hourglass proactively at any time you have priority.

Sequence:

• Play Zhonya's Hourglass from hidden (for 0 cost) when you have priority
• It resolves onto the board as a permanent
• Because gear cannot exist unattached at a battlefield, it is immediately recalled to your base as a corrective action
• It remains at your base, and its replacement effect will automatically save the next friendly unit that would die

Nuances:

• Leaving Zhonya's Hourglass hidden carries risk: if you lose control of the battlefield, hidden cards are trashed without applying their effects.
• Playing it proactively ensures it survives battlefield cleanup and will be available to save a unit later.