Usually INTJs are capable of creating a much more thorough and intellectual consistent idea framework than Lucas consistently shows. Really -- an NT reading his scripts should be laughing at the absurdity of how he puts ideas together. (His visuals are wonderful; but really, the story implementation is terrible.)
I would be surprised if he was an INTJ (like James Cameron, who is xNTJ -- perhaps it would do us well to compare Cameron and Lucas and note how they are distinctly different in their approach to these things). The ideas and dialogue and characterizations in Cameron's work are much more coherent and thorough than anything Lucas has done.
Lucas seems much more interested in creating archetypes and letting the story ride on their shoulders ("Look, these are the mythic archetypes and the story roles, so you should just accept them as valuable and well-done, ignoring how they're actually implemented") rather than actually "earning" the viewer's attention and winning them over intellectually like an NT would.
Another director who is possibly INTP is Christopher Nolan (Memento, the new Batman movies, Insomnia, The Prestige, etc.) -- and you see the same intricate locking there, the ability to weave together a pattern that is intellectually sound even if colored over with artistic flair. Very very different than Lucas' approach, who simply comes across as either an NFP of some sort or an SFP masquerading as an NF.