The problem with Hollywood satires like “Tropic Thunder” is that what happens in Tinseltown in real life is often more absurd than the wildest scenario a screenwriter could dream up. Tom Cruise has an extended cameo as a potty-mouthed Harvey Weinstein-like movie mogul in TT, but in real life MGM is insisting that Cruise will be taking the helm of United Artists now that his producing partner, Paula Wagner, has announced her departure as studio head. Or at least that’s what UA’s collapsing parent company, MGM, would like to happen. MGM wants to pump up its sale price by getting back into making movies after failing miserably at releasing other people’s movies (mostly the soon-to-depart real Harvey’s) — and can’t tap UA’s hefty $500 million credit line without the couch-jumping wacko’s participation. Cruise, meanwhile, is negotiating to star in a movie at another studio, even as MGM moved his first starring vehicle in years, Bryan Singer’s UA-backed Nazi docudrama “Valkyrie” up to this Dec. 26 after previously pushing the bad-buzz epic back from June to January. In a way, this turn of events is funnier than anything in “Tropic Thunder” not to mention the other upcoming Hollywood satire, Barry Levinson’s “What Just Happened” starring Robert DeNiro as a producer whose troubled film with a tempermental star (Sean Penn playing “himself”) ends up grossing $450 million. Don’t expect a repeat for “Valkyrie.”