We know basically nothing to about where the foxhead medallion comes from. But in addition to the evidence people have listed above, a couple things in context make me believe that the medallion would cover Saidin: (1) the medallion came from the Finns in Rhuidean, and it's implied that there has been no interaction between the Eelfin and humans since the Breaking when male Aes Sedai were normal, so when Mat asks for "a way to be free of Aes Sedai and the Power", there's no reason the Eelfin would believe he just means female Aes Sedai; (2) the foxhead has the ancient symbol of the Aes Sedai, both the dragon's fang and the flame, again indicating both Saidin and Saidar.