Force boss battles at Colosseum?

Just curious if there is any way to use enemies listed after Tonberries in the Colosseum. I know this isn't normally doable, but would there be a way to do a special event battle, like what is done when you bet the Striker and face Shadow? I wanted to use the unused WOB Umaro and Colossus bosses for this purpose but couldn't find any resources on how "special" Colosseum battles are handled.

Thanks!