I honestly doubt that Ash will call on Charizard simply to 'have a chat' with Mamoswine. Ash only called on his own Bulbasaur because the situation dealt with a fellow Bulbasaur thus the calling over was understandable.
Maybe Charizard can prepare an informative and interesting PowerPoint presentation to show Mamoswine, along with proper MLA citations and a well rehearsed skit showing the dangers of disobeying your trainer.
I agree with the fact that Charizard flying to a place in Kanto/Johto as opposed to Sinnoh is alot more likely. In fact, while I want the Oaks to come back for the league, I'd like it if Charizard and Squirtle don't come due to the fact Sinnoh is so far away. However, prior to the E4 Challenge if Ash goes that way, he can go pick them up first in Johto.