While I've not been able to find the location that controls/shows which mission is being played, I've found the location that modifies the special spawn list that is referenced for the regular hidden grotto funfest mission.
As much as I'll like to hope that the Special Funfest missions are in the game (the text are), they probably aren't.
If we think about if from a Wonder card's prospective:
1. Descriptions are in the game (which description displayed controlled by WC)
2. Subroutine on how to generate and gift a Pokémon is in the game
3. The species given isn't in the game (it's on wonder card) For all we know, it's the same case:
1. Description for missions are in the game (we know this)
2. Subroutine on how to generate the Hidden Grotto is in the game (they could just use the Noisy/Quiet subroutine)
3. The species encounter-able isn't in the game (we don't know this but who knows at this point...)
The first step anyone can take (besides this injection method), is to find Stunky/(forgotten name) in the garc.
edit: Apparently there may be a 4th wonder card mode. Not sure if it applies to special missions.
I can't seem to make it change the Funfest Title or control what I've 'received', tho.