If I hide Zhonya and my Poro dies triggering its Deathknell, can I play Zhonya in response to the Deathknell?

Yes — you can flip Zhonya's Hourglass in response to the Deathknell trigger, but it won't save the Poro.

Hidden cards gain Reaction when played from facedown (Rule 811), and the Deathknell trigger going on the chain puts the game in a Closed State — so you have a window to play it. However, Zhonya's is a replacement effect ("if a friendly unit would die"), so it has to be active before the death happens. By the time the Deathknell trigger is on the chain, the Poro has already died and been sent to trash (Rule 808.1.d.2), so the Zhonya's enters too late to replace anything. It'll hit the board as gear, and the Deathknell still resolves — you draw 1 if the Poro died alone.

One caveat: this reaction window exists because Deathknell formed the chain (and battlefields stay locked during an ongoing Combat). If the Poro died with no trigger to react to, cleanup would discard your hidden Zhonya's before you got a chance to flip it.