I don't understand your first point. Of course you need the bonus disc, there would be no point of using the hack mentioned in this thread or getting a Japanese R/S game to unlock bonus disc content without the bonus disc.
Dolphin can connect to some VBA emulators
Sorry, I didn't understand your statement at first, but again, what you suggest requires real hardware/software with the bonus disk, the japanese colosseum and the japanese gba cart. This method only requires the japanese bonus disc to get a legit celebi on the USA/PAL savegames. You don't even need a real hardware copy of the disc if using dolphin (but that's piracy, so let's leave the topic there).
Even more, you can use your own USA/PAL colosseum to get a legit celebi (without japanesse naming, some may think this makes it not legit, but at this point I don't think it really matters that much), no need for any jap setup.
About dolphin GBA connectivity: last time I checked, it was far from working at any usable state, the pokemon games where incompatible.
So... Has it been confirmed whether Celebi or Pikachu are obtainable from PAL Colosseum, as with USA?
Celebi is adquirable from PAL colosseum (my PAL celebi ready savegame is on gamefaqs, I had to play it just for celebi, since the completed pal save that was there turned out to be a USA converted save wich won't work on PAL games, or at least the multi language version I have).
I couldn't get pikachu, but I didn't mess much with that, also, I still don't know how are you supposed to get it. I messed with dolphin and the jap disc at the time, but didn't get it either, so I might be doing something wrong.