a less important note
We are a CLAMP-based game, and when it comes to AUs, this is always fun to play out - Body memories!
CLAMPverse has a concept that says even if the mind/soul/heart forgets, the body can remember. It comes into play at several points in TRC and Holic, from things like Sakura moving to kiss Syaoran's face to comfort him while not remembering their closer bond, or a heartless-at-that-moment Syaoran being unable to deal a fatal blow on Sakura because his body freezes up.
Of course, during an event like this, it's easy to see how it'd apply. Maybe your charas end up fighting in this event, but their usual bond is a close one, so they can't manage to truly hurt each other. Maybe the characters are just (quite literally) drawn to each other because they usually have close CR, despite not being that familiar in this scenario. Perhaps certain gestures from others feel nostalgic although they shouldn't. Etc, etc. Go wild with it.