You could do something like this:
BallHandlers::ModifyCatchRate.add(:MIRAGEBALL,proc{|ball,catchRate,battle,battler|
catchRate*=3 if battler.pbHasSpecies?(:CELEBI) || battler.pbHasSpecies?(:MEWTWO) || battler.pbHasSpecies?(:ENTEI)
next catchRate
})
You'd have to individually add each...