Could if they wanted to. If private Japanese investors, e.g. through mutual funds, decided to buy $1 trillion in US stock at the present low level of the dollar, they could have most of the US financial sector. Frankly, for a song. It'd earn them great gobs of cash in the future and fund a lot of happy golfing retirements, and I for one would be all for it.
At present, every housewife in Japan instead keeps a few million yen in physical notes stashed in every hollow object in the house...