Interesting question. I'll only answer as if this question was a driver full time, but it should be the same for combo people.
I assume you can be laid off. Understand that they can keep laying you off for 4 days and call you on the 5th......YOU need 5 continuous lay off days to then "bump" 2 lower seniority part timers. I would ask to NOT be called in so I could qualify for the 5 days... And if they do call you on the 4th or 5th day...Don't answer the phone....
Again talk with your Steward or leave a message with your business agent.