I've tried PikaSav and Metropolis Editor and it seems neither of these are compatible with the Japanese versions of the Gen I games. It would be very excellent if someone could develop a save editor compatible with the Japanese versions of the game, as it seems the Japanese versions of the game on virtual console are the only "legitimate" way of obtaining a Mew. Mew was distributed to VC Gen 1 games on the 3DS, but only on the Japanese versions. Since we don't have a Japanese editor, there doesn't seem to be a way yet to edit a Japanese save (which you would extract from a 3DS using SaveDataFiler) and inject the event Mew distribution.
Japanese pokemon from Gen 1 games are also incompatible with Non-Japanese/European/English USA pokemon, as trading a Japanese pokemon to a US game would cause the game to crash. So it would be nice to have a SAV editor exclusive for the Japanese versions of the game (Japanese "Aka", "Midori", "Ao", and "Pikachu").
If there's someone out there who can develop one, please let us know! If anyone knows of an already-existing save editor, even better! This seems like Western civilization's only hope for obtaining access to a Gen 1 legit Mew :'(