The concerns about persistent deflation are that households will decide to defer consumption expenditures in the expectation that prices will be significantly lower in the future than at present and that the economy will enter into a debt-deflation spiral. Such responses are highly unlikely in the case of small price declines over short periods of time.
To repeat, I suspect that operating on rates on longer-term Treasuries would provide sufficient leverage for the Fed to achieve its goals in most plausible scenarios. If lowering yields on longer-dated Treasury securities proved insufficient to restart spending, however, the Fed might next consider attempting to influence directly the yields on privately issued securities.