If I kill my Glasc Mixologist while I'm attacking into an enemy battlefield, can I play the unit I get from the Deathknell trigger to the battlefield I'm attacking?

No, you cannot play the unit from your Deathknell ability to that battlefield.

The Ruling

According to RiftJudge FAQ #9479 and FAQ #10112, when you move a unit to an opponent-controlled battlefield to attempt to conquer it, you are the attacker and you do not control that battlefield. You only establish control after the showdown is resolved and the battlefield is successfully conquered. Because you do not control the battlefield, you lack the permission to play units there via Deathknell.

Sequence of Events

1. Combat Damage: You attack with Glasc Mixologist into an opponent-controlled battlefield. Damage is dealt, and Glasc Mixologist dies.
2. Deathknell Trigger: Glasc Mixologist's Deathknell ability triggers and is added to the chain as a pending item (Rule 428.1.a.1.b).
3. Resolution Attempt: When you attempt to resolve the Deathknell ability, you are instructed to play a unit from your trash. Because you are the attacker and do not yet control the battlefield, you are not permitted to play a unit to that location.

This differs from a defending player, who already controls the battlefield where the combat is taking place and is thus permitted to play units there via Deathknell (FAQ #9149).