I just wonder if this'll be a single instance, or if we'll get to join her more than once.

I'm personally pretty thrilled. I'm not mad that we have an all-male main party, since the theme of brotherhood is an interesting take for FF to take, but I have to say, Larsa as a guest character was one of the best parts of the game, just going to show that guests can be incredibly important to a story, awesome characters, and just being overall very memorable even though they're not in the party.