Remember Egwene? She was held prisoner in the Stone, entered T'A'R, accomplished a Shield on a Black Sister and returned to her body only to find that the physical door was still locked. Moghedien could have done the same.

Especially since the most important information is already revealed to her: Nynaeve had forced Moghedien to enter T'A'R to go to Caemlyn to fight Rahvin, thus Moghedien could have been busy in T'A'R while sleeping, trying to find a way out of her predicament.