If I singularity a glasc mixologist can the units still be played to that battlefield?

Yes, you can play a unit to that battlefield using the Glasc Mixologist's Deathknell ability after it is killed by Singularity.

According to the RiftJudge FAQ #9329, when Glasc Mixologist is killed by Singularity, its Deathknell trigger is placed on the chain as a pending item before the unit is moved to the trash. Because the game pauses to resolve the chain, you are permitted to play a unit from your trash to that battlefield while the Deathknell ability is resolving.

Sequence of Events

1. Singularity is played and resolves, dealing 6 damage to Glasc Mixologist.
2. Glasc Mixologist dies and is sent to the trash.
3. The Deathknell ability is placed on the chain as a pending item (Rule 322.3).
4. Players have an opportunity to play [REACTION] cards.
5. The Deathknell ability resolves, allowing you to play a unit from your trash to the battlefield where Glasc Mixologist died.

Note: This interaction is permissible regardless of whether you have other units at that battlefield or if the battlefield becomes temporarily unoccupied, as the game pauses to resolve the chain items before final cleanup/control checks (Rule 187.4.c, Unleashed Rules Update).