During a Crusade, I think that system could work, but it should be the Pope or Papal Controller gets to set the objective as per CK1, but through the engine rather than an event. During the crusade, if someone of higher piety than the Pope comes along they could add an objective that would be considered a secondary objective. If there is already a papal and secondary, then the non-papal one is booted if another person calls for a target. The cost itself is based on how far away from the target it is. As soon as either objective is successful, the Crusade is called off as a success; the Pope or papal controller can also rescind his request and effectively canceling the Crusade.
The reason for papal/secondary is that hopefully the AI will try for the papal one if it thinks it can take that one and if not, it will go after the secondary one. Thus their is always one that is seen as more important.