Quote from Martinghoul:
Let me suggest a possible crisis endgame for you. Suppose QE is infinite, like you mention, and the time comes when, eventually, the BoJ owns the entire outstanding stock of JGBs (which were paid for by printed JPY). At that point, the BoJ declares JGBs worthless and burns them. In this case, the private holders of JGBs never experience the worthlessness of their JGBs (only the ccy suffers).
The reason I bring this up is that Japan has done this already in the past. Takahashi Korekiyo during the 20s presided over policies that were somewhat similar.