Hi,
Well, it's not possible to limit Generators to Material IDs used in Clipping area (where such spline is not used to generate the array, but only for clipping purposes). Instead, you can use Material IDs on the Spline used for generating the array (X Spline) and another spline for Clipping area.
Attaching a sample scene showing this work flow. Hope you'll find useful.
Best regards,