@creece This is how I modified a Saved Search's Filter Expression in a Workflow Action script:

var search = s.load({...});
search.filterExpression = search.filterExpression.concat([
   "AND",
   ["email", <http://s.Operator.IS|s.Operator.IS>, data.email],
   "AND",
   ["custevent_meeting_date", s.Operator.ON, data.meetingDate]
 ]);