The Committee - Seeking the Fruitful Void

Vincent Baker coined the term "fruitful void" here to describe the part of a game that is central to its theme and play experience but that isn't covered by the rules (e.g., The Mountain Witch is about samurais and honor but there aren't any rules for tracking honor points or using honor mechanically). The idea is wicked cool, and I can see how many designs have created a fruiful void that ends up driving play. Creating a fruitful void for The Committee is something I want to do, but execution is a bit harder that theoretical understanding.

The Committee explains the involvement of the characters in the expedition that they are reporting on by establishing that each character has a secret desire whose achievement requires the additional acclaim and recognition to be gained in going on the expedition and coming back with thrilling tales of their exploits. Of course, while these desires are "secret" to the other characters, all the players know the desires of the other characters so they can frame hazards that touch on them. At the end of the game, each player gets to narrate an epilogue for their character in which this desire is gained or lost, depending on the amount of Acclaim (in-game currency) they have accumulated.

Ok, so the desire helps explain why the character is going on another expedition into danger and its gain or loss ends the character's story, but how to use the desires during play? Currently, the desires of the characters are story elements that the character gets for free (usually creating a story element costs one point of Acclaim). When another player reuses your story element when acting as Opposition for another character or providing reflective narration at the end of their own character's scene, then you get a point of Acclaim and so do they. In addition, you can choose to "cash out" of your story element and receive back the initial point of Acclaim you spent to create it. This keeps the system nice and streamlined, but it doesn't seem to make desires as important as I want them to be. The local playtests I've done used the desires but didn't see them driving play as much as I hoped. Of course, early playtests spend so much time finding rules kinks that focusing on larger goals of play can be difficult. Still, I'm not satisfied with the current treatment of desires.

Another thought I've had was to allow players to call on desires during a scene by making a narrative aside describing how their desire influenced their actions in the scene for good or ill. In exchange, the player gets to roll a die or two to help him successfully overcome the scene's hazard. Joshua BishopRoby's "thematic batteries" in Full Light, Full Steam may end up being inspirational here, since I'd like calling on your character's desire to have both positive and negative effects that you can manipulate to maximize your benefit when it really counts.

So does such an approach enable the desires of the characters to drive play more effectively? Or should I not focus mechanics on the desires at all? Which does the best job of trying to create a fruitful void in the game? Testing of both options will tell the tale.