Central bank governors aren't supposed to participate in initial public offerings. But Haruhiko Kuroda could just be the main winner from Japan's biggest in 30 years.

For 31 months now, the Bank of Japan head has struggled to gain traction with unprecedented monetary stimulus efforts. The $700 billion of Japanese government bonds Kuroda buys each year is proving no match for a two-decade deflationary funk. One reason is that the BOJ is literally having a hard time finding enough JGBs to buy. The main financial asset held by banks, pensions, insurance companies, universities, endowments, government institutions and households are in shorter and shorter supply in the secondary market.

That's why Japan Post's $12 billion IPO couldn't come at a better time for Kuroda. The giant savings bank inside a state-owned company employing 400,000 people has long been the biggest hoarder of JGBs. Now, as the privatization process unfolds, that bank will be under pressure for higher returns. Expect it to accelerate sales of JGBs, municipal and other public securities that as of April amounted to almost $1 trillion. April is important because it marked the second anniversary of Kuroda's failing deflation battle.