According to biographers (and friends like Coppola) that's indeed the MPAA and the movies unions that inspired Lucas his Empire back in the mid-seventies. That, and Nixon, and the Third Reich. Lucas doesn't like Hollywood, and Hollywood doesn't like Lucas. He even broke from the Directors' Guild before ANH because of the union regulations that would have forced him to put opening credits on the movie.

But the success of the first movie has freed Lucas from all creative constraints but his own, essentially financial. Lucas financed the movies with his own money, and hired himself a producer (McCallum) who had the reputation (credentials, from his work on the Young Indy series and the SW restoration project) to be able to keep production costs as low as possible without compromising quality, and who wouldn't let him go over-budget. Same on ILM's side - Lucas didn't want them to lose a cent working on SW - Lucasfilms paid full price for the services of ILM for SW (though Lucas "cheated" by investing from his other pocket so ILM developped the technologies he needed for the prequels).

Fox Studios are not even involved in any way with the prequels (or ESB/ROTJ). The studio behind the movies is Lucasfilm, Lucas' own company, and virtually all the service providers involved (VFX, Sound etc.) were part of Lucas' empire. He's the writer-director, the head of the studio and the head of the services companies. He's the head "movie exec" on his own films, a one-man mini-Hollywood.

The Star Wars prequels are the biggest independant movies ever made. They were produced with the same creative freedom and with the same process as auteur cinema, just with the budget to produce about anywhere from 500 to 1000 such movies.

It's strictly the distribution branch of Twentieth Century Fox that's involved in SW. They have zero say in the creative process (no more than they do for any movie, not even those made by their Fox sister-company), they have zero say even in the marketing of the movies, which is also the responsability of the studio (here, the marketing branch of Lucasfilm), and most importantly, they had no money invested in SW. Lucas merely hired Fox' services to put his movies in theaters, to negotiate his foreign deals, and later for distribution on the home market worlwide. Unlike most independant movies, Lucas didn't sell his movies to his distributor, he rather paid for the distribution services (a fixed price or a small percentage, no one knows). And Fox distributed the prequels under several constraints from its client, for instance Lucasfilm insisted for TXH-approved theaters, meaning screening rooms with the THX technology (yet another branch of Lucas' empire) in place and that had passed the THX bi-annual inspection.

Not that Lucas wasn't challenged creatively by his collaborators. He was, and he wanted to. But he had the last word on everything. Those "exec" that could force him to change this or that simply didn't exist.