> If you're able to predict with high confidence how this effects the yen/usd rate, then you can make a huge amount of money on fx arbitrage.

Thats not true. It could be the case that the effect of interest rates on exchange rates is obvious to everyone, and therefore it is already priced in.

Interest rates are not an obscure, arcane thing. Lots of people in finance know how they work, and know how they effect various parts of the economy.