Unfortunately given Pikachu's disdain for being in a Pokeball, I don't see how that's possible. Granted Pikachu being out for the running and water race may be cheap as it is, but Pikachu and another Ash's Pokemon during the final lag of the triathon sounds really cheap. I guess Ash assumed that he didn't want to be unfair to the other players so he just chose Pikachu, since it was convienent.I wish Ash uses some1 else for a change.
Though I also wish he would use something else, I don't really see the problem of letting Pikachu stay with Professor Rowan and having Ash use someone else.
But then again maybe they don't care (Pikachu is an exception since people knows it hates being in a Pokeball. Remember when Ash had to climb the mountain to face Danny, and how he wasn't able to use any of his Pokemon but Pikachu was there. He wasn't disqualified for just having Pikachu there.) and the only reason why were assuming it's Pikachu, is because its the only one we saw in the preview. Ash could very well be using a different Pokemon than Pikachu (but he probably isn't).