Advance frames on an Japanese VBA. Make a save state, capture Mew, make a normal save, load the .sav on an English VBA, check your frame and work from there.
There are no warp codes (or any) for Japanese roms as far as I know, which means you have to go back to English roms each time to use desired codes (Like checking your frame). This means you'd have to warp to the island on a English rom, load the .sav on a Japanese rom and abuse it on a japanese one.
For further questions please ask someone else as I have exams which means I don't have time to hang on Smogon and besides I hate explaining the same shit to people all the time. If it wasn't for Nicksor I would not have posted here most likely.
And on a sidenote, you can't load English save states on a Japanese rom or vica versa, only .sav files.