If you use LeBlanc's legend to mirror a unit on Harnessed Dragon, does the new one count as played and reactivate the 'When you play me, kill an enemy unit'?

No, the new unit created by LeBlanc’s legend (Deceiver) will not trigger the "When you play me" ability of Harnessed Dragon.

Why it does not trigger:

1. "Played" vs. "Created": In Riftbound, the "When you play me" triggered ability specifically requires the card to be played from your hand. LeBlanc’s legend ability creates a unit token (the "Reflection" token) and then applies a copy effect to it. Even though the legend ability uses the word "play" when placing the token on the chain, this is a token creation effect, not the act of playing a card from your hand.
2. Copying Abilities: When an object becomes a copy of another, it gains the copied unit's abilities (in this case, "When you play me, kill an enemy unit"). However, since the token was never "played" from your hand, the condition for that triggered ability ("When you play me") is never met.

As clarified in RiftJudge FAQ #10362 regarding copy effects:

"The 'When you play me' trigger only triggers at the specific moment the original... card is played from your hand. When LeBlanc creates a copy... that token is not being 'played' from your hand in the way required to trigger that ability. Therefore, the effect will not occur."

This is my interpretation based on the standard rules for copy effects and triggered abilities; no official FAQ exists specifically for the Harnessed Dragon interaction, but it follows the established ruling for LeBlanc's copy effects.