Can Zhonya be played in base to save a unit on a battlefield?

Ruling: Yes, Zhonya can be played in base to save a unit on a battlefield, but not as a hidden spell.

Nuances:

• Playing it hidden in base allows you to use it as a reaction for 1 Power instead of 2 energy
• Playing it hidden denies information and allows bluffing other hidden spells
• Playing it hidden lets you choose when to use it (so it doesn't trigger early for units you don't want to save)
• If Zhonya is reacted with at a battlefield but not consumed (e.g., unit removed with Gust), it goes to base and becomes useable for any units (though designer intent was for it to remain locked to the original battlefield)