Parked my SK in the "wry" imp room. He popped several times in an hour before dropping the dagger hilt. Next camped the "a sly kly imprecator" room (Laboratory) for hours before realizing that Jaxun is right and parked two other toons in the "wry" room and "shy" room in order to clear out placeholders. Took around 8 pops for "sly" to appear and drop the jagged metal shard.
So yes, park a toon in each imp room to clear placeholders in order to force a pop.