In my opinion, I feel like if something like that were to come up, Ash would not be the partner. Either Serena asks Ash to, and Ash refuses because he doesn't know much about performing and doesn't want to end up ruining it for her. Or, Serena knows that Ash doesn't know a lot about it and his dancing skills (As seen in XY044, I think) don't really help, and just doesn't ask him. I think Shauna would be the candidate, or someone else who knows a little more about performing.
Ash is focused purely on Battling, and Performing is far from his forte. So if Serena was smart, she would choose someone else as her partner.
But that said, if such an episode was to come out, there could be an episode with Serena training Ash in performing so he could be her partner. Nothing is set in stone, especially in this series. But as usual, that's just my opinion.
She would ask Shauna. Ash just isn't about the showcase life.
Plus, I think the showcase is a girls only type thing iirc
To be honest, what do we really know about Showcases? Can we really say its just a girl thing?
And you guys say that Ash might not be interested but we can say the same thing about him and contest but he still participated in one or two. (dont remember the exact number)
I dont think anything is stopping him from wanting to try it out for himself at least once especially if it is to help a friend. =)
Edit: I guess theres strong evidence of it being just a girl thing considering they are competing for the title of Kalos Queen and no guys have been seen just yet.
SO i guess this makes m point moot. XD