Wasn't it Seaine who tried organizing the Aes Sedai during the Seanchan raid on the Tower by setting up a base of operations in the lower section? Why would she do that rather than let the Seanchan create more chaos?
That was Saerin btw. Saerin is the brown sitter. Seaine is the white one. A white sitter would suit Mesaana´s tastes better also imo.
Mesaana is described more like a brown than a white at the beginning of the book: "Sometimes Graendal thought Mesaana had gone over to the Shadow because it offered a more interesting opportunity for research."
Danelle is the leading candidate in my opinion - she is a Brown with blue eyes who was one of Elaida's party that takes Siuan into custody, and she was one of the first people Alviarin passes between Elaida's quarters and her own. That being said, Danelle might be too obvious.
I thought of Silvania when I was reading too, but I'm not sure how she would have dodged the Black Ajah oath, unless Chosen isn't considered Black Ajah. It would be a good twist though. Her actions would make sense as well - seeing Eliada's downfall as inevitable, she throws her weight behind Egwene to put herself in a position of power. Also, before she became Chosen, she was a teacher, which would be similar to her Mistress of Novices position.
I hadn't thought of Laras but that would be a good twist too and a way to escape re-swearing on the oath rod to evade detection. Laras is still a mysterious character - I remember Egwene noting that she has the look of an Aes Sedai and wondering, 'who is this woman'. Laras as Mesaana, might also have seen Egwene winning the battle for Amyrlin against Elaida, which would make her actions to try and 'help' Egwene escape understandable (i.e. if she supporrted Elaida, she may have thought that Egwene would do less damage outside of the Tower).