Serpent Kingdoms is silent on the matter of medusae and maedar. Seems that FR is reluctant to make them snake people.
Reading the Ecology of the Maedar in Dragon 106, it's worth noting that the tale Elminster tells is set in the caves of "the wild North", and the adventuring band that encounters the maedar sets out from Waterdeep. We could spin some yarn of our own about them, then. They might be most commonly found in the Crags or in the Spine of the World. Maybe they occasionally slip into Waterdeep to buy and trade items of a more civilized nature (note how medusae always have such nice robes, not to mention excellent bows). Have they formed a grudging alliance with King Obuold Many-Arrows? Are glyptars being secretly sold by an unscrupulous gem dealer in Waterdeep? It looks like we're free to use our imaginations here.
Demiurge out.