Can you react to playing your own unit to trigger its Legion effect?

No, you cannot react to playing a unit to trigger its Legion effect.

According to RiftJudge FAQ #1526:

• Legion triggers do not go on the chain if the Legion condition is not already met when the card is played.
• You cannot respond to activate Legion after playing the card.

Furthermore, as established in RiftJudge FAQ #2971 and #7795:

• Playing a unit does not grant priority, and units leave the chain immediately without allowing responses.
• Because you cannot react to the act of playing a unit, you cannot create a window to play another card to satisfy the Legion condition before the unit's "on-play" effect checks for it.

Summary of the interaction:

1. You play a unit.
2. The game checks if the Legion condition is met (i.e., have you already played another Main Deck card this turn?).
3. If the condition is not met at that exact moment, the Legion ability does not trigger and does not go on the chain.
4. Because playing a unit does not open a window for reactions, you cannot play a different card or effect to "turn on" Legion after the unit has been played but before its ability resolves.