What if the one Piplup that walked over to Ash & Co. in Going For The Gold! was Dawn's Piplup? Of course, Ash and Pikachu would immediately greet it, recognizing it when Dawn appears. From there I have nothing other than the fact that they need to high five.

Yes, it was directly inspired by the small mentioning of Dawn's Piplup in said episode.

Also off-topic, but I accidentally came up with a Pearlshipping (or atleast Dawn encounters Ash) in Alola fanfic title while trying to come up with a title for a Clemont & Bonnie in Alola fic. The title is Every Dawn Has A Dusk so feel free to use it if you'd like.