I'm not going to join the Cleef pile-on, but I will tweak your theory a little bit:
(1) There is a branch of the military in charge of loaning helicopters, airplanes, and other equipment to Hollywood, and in recent years it's become politicized, with military personnel vetting scripts before allowing the equipment to be used. This isn't exactly an official censor, but if this military office doesn't like the contents of a script, they can demand changes or just refuse to loan the equipment. Sometimes this is tantamount to killing the movie because military equipment can be so cost-prohibitive. As far as I know, this sort of thing has been going on for years, though not necessarily in an overtly political way, like the bit about Francis Ford Coppola using the Filipino military in Apocalypse Now that made it into the documentary Hearts of Darkness. As with other matters like these, the influence is more subtle than out-and-out propaganda, and self-censorship is usually the order of the day - a producer who's self-conscious about offending the military liaison is more likely to demand changes than the liaison himself.
How do I know this? The Marine (I think he was a marine) spokesman in the documentary Control Room used to have this job and he went on Democracy Now and talked about it. It's worth tracking down.
(2) All Hollywood movies are propaganda pieces, but the ideology they put forward is not liberal or conservative or even corporate (though a movie can contain any of these agendas). The ideology a Hollywood movie puts forth is Hollywood itself: the heroic individual is what really changes things, love will conquer all, Hollywood is extremely important, and so on. I thought Tropic Thunder hit this last one really hard, which is one of the big reasons I wasn't crazy about it.
It's an awfully effective propagator of its own ideology, much more so than anything emanating from Washington, DC, or from the NYT, WSJ, CNN, or Fox.
