If the Saliszi had soulsteel in the Godsfang mountains, it likely wouldn't be too far of a stretch to say that the secret of its making still lies in that region, unless of course, they acquired it after leaving the peaks. If they did find it after leaving, then who's to say why they left? I am warming up to the idea that the secret lies within the mountains themselves, however.

And as for living under the Saliszi Empire, well the Godsfangs are as much in Drathan territory as they are in the Saliszi's, and the Saliszi Empire has not been able to conquer the Dratha. Maybe they weren't able to make any offensive move against the people of the mountains.