Not exactly unused Pokémon, but I believe I have found team info for two unused trainers (DAGUR and EVAT) for Pokémon Colosseum in the Colosseum Battle -> Solo Battle -> Mt. Battle Vs 100 -> Double Battle option while using a registered team from a GBA save. While running tests on my PKX Extraction application I kept finding the same 12 Pokémon in the RAM stored between the NPC trainers team and the story mode party team. Initially I thought it was an error where the application will sometimes recognize data that isn't a Pokémon as a Pokémon for games without a checksum. However, when the application makes an error like that the Pokémon usually has a garbage nickname or OT and a random assortment of moves with other aspects of the Pokémon not being correct. In this case all of the Pokémon data is valid (though not legal) with no viable garbage data. Additionally, the trainers names do not match any known trainer within the games as far as I can find on Serebii and Bulbapedia. This has lead me to believe these two Pokémon parties belong to unused trainers for this battle mode in the Colosseum.
Usused Trainers.zip