I remember doing the truck trick in Pokemon Blue. You fight the S. S. Anne captain to get HM01 (Cut, which enables you access to Lt. Surge's gym and the rest of the game), but rather than exit the boat you instead deliberately lose to one of its passengers in battle, so that you're magically spirited to the nearest Pokecenter without triggering the boat's departure once you leave it. That way access to the docks won't be revoked, and you'll be able to swim right towards that mystery truck once you have Surf and Stregth.
I tried all that, and while I was surprised enough to find the fucking truck, there's nothing to be done with it.
I tried a bunch of other tricks as well. Most of them were on Blue and usually involved Misty's city and the Nugget Bridge. Also something about the daycare outside Goldenrod in Gold. I forget what exactly.
They're all make belief. I think the only way of getting Mew was through a tournament or something like that. I refuse to believe anything else, I don't care if you post a video about this totally tried and true method.