Regarding Ageto Celebi, I looked into it. While the initial Celebi received is added to the Colosseum party as I had originally thought, the remaining 48 are sent directly to a GBA game.
Also found a video:
Sounds and looks like a multiboot to me!
Unfortunately, I can't find any details on Colosseum Pikachu. I guess we'll have to see.
So at the moment, we need to extract pokechan.bin from a EUR Pokemon Channel iso, and whatever file in the JPN Bonus disc that should contain Ageto Celebi, and possibly Colosseum Pikachu. I'm not familiar with extracting files from isos, but I'm looking into tools.