I agree with everyone else above that that is no extra merit in serving at Mass. To assist sounds like doing something beyond mindful participation but I’ve not found*anything in lists of indulgences about extra graces; plus I realized how unrealistic it was. I would often go to daily Mass at chuches that were not my home parish. Some parishes might rely on God’s Providence to find readers, etc for weekday Mass but many plan or at least rely on regulars. I guess I think those who use their lunch hour for Mass should merit more than those with fewer obligations (I.e., those with more time on their hands) should merit more.