CROSSOVER OF THE WEEK!
Cartman was in Hollywood, hoping to force FOX into canceling 'The Family Guy', allegedly to keep the terrorist placated as the cartoon was supposedly going to depict the prophet Mohammed in the show. (Actually Cartman just wanted the show off the air because he didn't find it funny.) Stan followed him out there to stop his campaign - his argument being that if the network gave in to the Islamic terrorists, they win.
Why Bart Simpson was originally in Hollywood is unknown, but not out of the realm of reason. He's been to New York City, Australia, England, and he even washed up on a desert island when the school bus crashed into the ocean.
And there's always the possibility that Springfield isn't too far from Hollywood; we've never learned in all the years it's been on the air exactly what state this particular Springfield is in, although it does need to be near the ocean, apparently.
Now, a cynic might claim that Bart was there on the FOX lot just because he was an "employee" of the network, but that takes all the magic out of it. And considering the fact that the two-part 'South Park' episode was already one big Zonk! by going after 'The Family Guy' as a TV show rather than accepting it as being integrated into the same universe, who needs that headache?
There was no doubt that it was Bart Simpson, even if Nancy Cartwright wasn't providing the voice. Any vocal differences could be attributed to a cold, anyway. Despite the alteration in his appearance due to the animation style of 'South Park' as opposed to that of 'The Simpsons', Bart looked just like he does in his hometoon. The red shirt, the short pants, spiky hair, and the yellow skin coloring. He was even carrying his green skateboard (using it as a weapon) and he said one of his most famous of catch-phrases: "Cowabunga!"
Stan and Cartman never addressed him by name, not that he introduced himself on screen; they only referred to him as "Kid". So that leaves the question of his identity open for inference, and I'm taking the option that best suits Toobworld: it WAS Bart Simpson.
And even though it's not my birthday, I'm applying my mantra of "What I say, goes."
As the Bart-Man and Cartman would say -
Don't have a cow, man. Respect my authority!