If I remember correctly the following statment also applies to all media star wars related. It has to basically fit into or be apart from alot of the estabilished canon for the films. KotOR basically got offered on which period they could develop in. I am pretty sure same thing will happen with the MMO. Lucas has another company that keeps track of all the various storylines throughout all the different media and makes sure they dont conflict as much as possible.
So while Lucas prolly wont dip his hand into the plot. It is still gonna have to follow some rules for the setting.George Lucas retains artistic control over the Star Wars universe. For example, the death of central characters and similar changes in the status quo must first pass his screening before authors are given the go-ahead. In addition, Lucasfilm Licensing devotes efforts to ensure continuity between the works of various authors across companies.Elements of the Expanded Universe have been adopted by Lucas for use in the films, such as the name of capital planet Coruscant, which first appeared in Timothy Zahn's novel Heir to the Empire before being used in The Phantom Menace. Additionally, Lucas so liked the character Aayla Secura, who was introduced in Dark Horse Comics' Star Wars series, that he included her as a character in Attack of the Clones.