that Moiraine has been to SL before. She seemed familiar with the city, and the problems inherent to the locale.
I also suspect, and this ties in with my SL theory, that, one day, I swear, I'll post, that SL, Mashadar, and Mordeth are more deeply touched by the Shadow than we know.
If this is so, then her Wards would just have to be minor variations on Wards known and used against the Shadow, and those would be taught to everyone in the WT.
She asks Vandene/Adeleas about the city when she stops at during TGH and from their conversation it sounds more like SL history is part of the education received by Novices/Accepted.
I suppose it is possible that she and Lan went there during their search for The Dragon, but given how reluctant they were to go into the city in EoTW it seems unlikely that they would have ventured into the city for anything other than an extreme emergency.