SCL is supposed to be open from 6:30AM to midnight 7 days a week. You should be able to set a 6:30AM return. Almost all airport locations have after-hours key return boxes. I would just set the return for 6:30AM (we tried and it seems to work for us) and then ask at the rental counter when you pick up about returning before they open. There would have to be some arrangements if flights leave earlier.