Time to release my random creativity
I think the idea of Itsuko arranging the V-Day event does work, at least somewhat. After all, it would work with the fact that there's almost no way to make Kyonko even think of giving chocolate to anyone in her current situation. She wouldn't, but Itsuko's plot (sounds creepy...) would leave Haruki under the assumption that he
did receive chocolate from Kyonko. Of course, that would be less cute than Kyonko actually giving him some, but with her personality, it is indeed a bit hard to come up with a reason for her to do it.
As for why Haruki wants to go treasure hunting in the first place... Well, Itsuko could just tell him that she knows of a legend involving a treasure buried in the location. However, she would do it in a rather obvious way; in other words, intentionally give Haruki the impression that she's not telling him the truth. And since her suggestion involves all of them going, most notably Kyonko, and Haruki has been despairing over whether or not he'll get anything, he would pretend to go along. Of course, in the end he'd just announce how he saw through the plan from the moment he heard of it, which would give Kyonko a way to realize what was really going on.
Maybe not the best idea in the world, but that's one way I can imagine being even remotely possible. However, the whole Mitsuuru part does probably mess it up a little. The only reason I can imagine for him to want to be secretive is that he too assumes that Kyonko is in the plot (and thus thinks she knows what it going on at the treasure hunt), and doesn't want to... err... ruin it for her?
Unfortunately, that's all I can think of. Hopefully it'll at least inspire someone into coming up with something better, as that might be (more than) a little too complicated.