While Bank of Japan Gov. Haruhiko Kuroda has underscored how much work still lies ahead, when the central bank does finally hit its inflation target and exits stimulus, it is likely to face huge balance sheet losses.

Kuroda has brushed off the need to discuss the details of what happens when his unprecedented asset-purchase program comes to an end, but economists are already debating the matter.

Adding to the uncertainty, there is no provision in the central bank law for the government to inject funds to repair its finances, if it should come to that, as some suggest. Less dire scenarios see the BOJ running down its reserves and capital, and largely operating as normal while taking years to pull its balance sheet back into the black.