Going under the impression Saturday air is still PT in your center, they must ask PT qualified, ask FT qualified, force PT qualified, and force FT qualified. The purpose of PT cover drivers in the Souther Supplement is to cover vacations and vacancies of FT drivers. Saturday air is a signup basis only and they cannot force you unless they have gone through the first procedure in seniority order.
If they get to the point of forcing qualified PT cover drivers after asking FT and PT guys, you can DQ yourself from PT cover driving, but that also means you don't PT cover during the week.
Hope this helps.