Is Norine Dovarna: "Big-eyed and nearly as dreamy as Danelle at times, and as friendless, she resented Alviarin; if the Keeper came from the White, in her eyes it should have been Norine Dovarna." (ACoS, prologue)

That she doesn't have friends, doesn't like Alviarin (maybe just to avoid contact), is dreamy (to hide discrepancies to the real Norine?) and her striking eyes could point to Mesaana.

The only problem is that Alviarin knows Norine and should normally already have figured out why Mesaana's appearance and voice seem familiar to her (if Messana looks and sounds a bit like Norine). On the other hand, Alviarin maybe has other things in her head with SH's orders and Silviana's punishments.

There is also one other scene with her & Alviarin in KoD:

“You’d grimace too, Ramesa, if you were being strapped every morning before breakfast,” Norine said, much too loudly and plainly meaning for Alviarin to hear. Ramesa, a tall slender woman with silver bells sewn down the sleeves of her white-embroidered dress, looked startled at being addressed, and likely she was. Norine had few friends, perhaps none. She went on, cutting her eyes toward Alviarin to see whether she had noticed. “It is irrational to call a penance private and pretend nothing is happening when the Amyrlin Seat has imposed it. But then, her rationality has always been overrated, in my opinion.” (KoD, prologue)

Maybe Mesaana is testing, if Alviarin has recognized her.

Egwene also notices Norine guarding Leane in KoD, ch.24:
"Norine, lovely with her large liquid eyes but often as vague as any Brown."

The comparison to the Brown could be another clue pointing to Mesaana. It also makes sense that Mesaana would be vague, because she doesn't want to slip.

Well, I think Norine being Mesaana is my bet for the moment.