Does Yahoo have anything where they will automatically adjust your roster for you if you are unable to make it to the computer for an extended time period? If you know anything of the such, please let me know.
Also, I do know that you can adjust your roster for the future, but this is such a long and boring process.
There is no way for it to automatically fill for you. If you're going to be away from your computer for an extended period of time, you either have to a.) do what you said and fill out your lineup for the future, or b.) not sign up for a fantasy baseball team at all, because you will be an inactive manager who will bring down the competitive level of your league.