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I don't know, sorry.
If Japanese Faraway Island Mew isn't 'shiny locked' (made to never be shiny), then you may be able to get a shiny one.
If it isn't shiny locked, I have doubts playing on high speed would prevent you from seeing one, because I've obtained different natures that way in English Emerald (through different personality values), and shininess partly depends on the personality value, however, I haven't confirmed different natures on Japanese Emerald because I don't know how to get there.
One note. Don't use a VBA movie, because using the same inputs and starting point will always make you obtain the same Mew.
Edit: You may want to change the title to "Japanese Emerald Mew".
Edit 2: What codes did you use to get to Faraway Island in Japanese Emerald? This site says codes are (M) CADD22F0 ADF77526 and C88758CA B308BAA7, but I can't get them to work.
Faraway Island Mews can be shiny. Here's a video of one being caught:
I actually heard somewhere is that if you have a Japanese Emerald cartridge that someone brought to Pokemon Festa 2005 with, not just can the Mew you encounter be shiny, but I heard it's the only time any shiny Mew would be legit. I could be wrong, though.
I also confirm it can be shiny.