Monetary policy in short-term private assets, such as commercial paper could not only lower private rates relative to government rates but also allow the central bank to work around an ailing banking system. Finally, open market operations involving foreign financial assets – effectively unsterilized intervention in the foreign exchange markets – could, in principle, affect exchange rates.
However, there is really no substantive difference between sterilized and unsterilized operations when the short-term interest rate is already zero. One way in which open market operations in other assets might affect other rates or other asset prices would be if there were relative asset-supply effects determining longer-term private rates and exchange rates.