Well, not knowing the plot down to the letter, does the player only find out about the final boss (King Zeal, yes?) right before they're forced to fight him? If not, I don't see why something couldn't be implemented like the 1999 A.D. bucket. Just have it disabled until the player learns the identity of the final boss.
Alternatively, if there are some lead-up bosses to the final encounter (IE: like Final Fantasy VI, let's say) then grant the player the ability to use the bucket whenever they want. They'll get creamed before they learn the identity of the final boss anyways.