I can see this being difficult.

Let's think currently the coding allows it as a party member to allow you to gain xp, and your chocobo to gain xp, while targeting and fighting for you reacting to you.
Plus you have the timer of the chocobo that is dwindling. Now the timer portion I can see they could use the same coding for that as they do when mounting, or in the major cities. However, the portion I think that's difficult is how would it be able to differentiate between a PC in the party and the Chocobo, since the chocobo's names can be created by the character as well. Also to prevent the coding from the chocobo to accidentally appear in the dungeon as well or cause a 5th/6th person to come in. Now let's take into consideration for the "pre made" parties. Don't forget they may use chocobos as well, so doing the coding to include those party members chocobos. While it may appear easy, there is many moving parts and some little tiny things we forget.