I would think that at least each Ajah Head knows the AS in their respective Ajah. Certainly there are also other AS in the administration of the WT or each Ajah, who know, who the Sisters in the WT are. After all, otherwise there couldn't be the record of who was killed, captured and BA after the Seanchan attack.

However, even if it were possible for Mesaana to create a new AS identity in the WT, Mesaana can't be one of the AS we get to know in the series, because they are known to others and obviously have a past (e.g. Danelle).