Good point Bossk, I hadn't thought about that.
I have an idea.
He could do the period of time 4000 years before Yavin, but he'd interfere with the 'Tales of the Jedi' comics (which, knowing Lucas, he wouldn't have a problem with).
In addition, there is a huge chunk of SW history during those 4000 years that no one (to my knowledge) has gone into great depth with.