How do I rejoin a game of 100% Orange Juice?

While playing an online game of 100% Orange Juice with my friends, I ran into an error which made me disconnect from the server. I tried to rejoin the game, but I could only find an option to spectate while an AI took my place.

Is it possible to rejoin a in-progress game of 100% Orange Juice?

You were pretty close.

When you join a lobby for a game in progress, you can only choose spectate so do that. Once you see the game, you can click the "Join" button at the bottom center then choose an AI to replace. The lobby owner has to accept your request to join. If he does, you'll take control of the character. It might take a whole chapter before you can actually take the AI's place, I'm not too sure on the logic for that.

Do note that you cannot take over a DLC character you do not own.