I thought Trey and Matt wrote all their own scripts, anyway.
But Cartoons make their episodes more in advance than live action series do, so a lot of these shows should be good to go script wise for a while, barring rewrites and the FCC.
Then shows like Robot Chicken and Family Guy don't look like they really even need an actual script to go by, so all Seth McFarlane and Seth Green would have to do is just do what they now: Insert random pop culture references and a little shred of actual story.
And the way Fox pre-empts their cartoon lineup, Simpsons, King of the Hill, Family Guy and American Dad should all have their current seasons already finished (or getting there) with next seasons scripts in the early stages.