I looked at the trash bytes for Mew, and I am fairly convinced after speaking with @Bond697 that mystry mew was generated using a distribution ROM. Most likely the team modified the existing multiboot rom code to generate them in batches of 5. We know it was a distribution ROM because the trash bytes of the first mew is consistent with other events (like 10 aniv) that came from multiboot roms. Bond researched the trash bytes which ended up being from lazily copying the pokemon's name with no regard for the data that came after the terminator. Upon inspecting what that data is, Bond found that some of the distribution code was copied to the stack for an unknown reason.
Our theory is that they made a button that adds 5 mews to the save and hit that button a million times. IIRC, in that save file the town was blocked by a tree, trapping the character in town. This leads us to believe that the original cart used to make the save file was most likely a dev cart that had manipulation capabilities. Also makes sense since they would have to extract the save for manufacturing.
For what it is worth, we have a range of trash bytes by observing the other mews. And of course when you xfer to XD/Colo or Pal Park, PokeTransfer, Bank, etc, trash bytes don't matter anymore anyways.