The futurama idea wouldn't be much of a crossover/tie in. I would much rather see GDT appear on the Simpsons as a guest star trying to plug Pac-Rim to theatergoers, now THAT would be amusing.
Now then again, that's me.
Or if fate provides, Pac-Rim will eventually have the same press and hype Avengers had (thanks to the awesome Pac-Rim teaser trailer) and people will flock to see Pac-Rim, even without a viral campaign.
If we do have a viral, I'm going to hope (Cthulhu willing) for a viral campaign as awesome as the Hellboy one.
Bored homo-sapiens-sapiens sitting in front of his laptop either cranking out useless streams of information, reading a book or watching a movie.