Hmm I'm not totally sure if this is supported, but int he past I remember using "[CustomEditor(typeof(ClassA),true)]" above the editor class to do this in my own projects. The "true" bit is supposed to enable a custom inspectors in child classes, but I think it has to be above the parent class. Try adding this to the header above SuperTextMesh's custom editor script in SuperTextMesh.cs, and I'll add it as a feature in the next update if it works.