Central banks through history have occasionally acted beyond a narrow inflation targeting remit to address acute financial stability crises in the past (as when the Bank of England famously secured permission in a moment of panic to exceed the limitations of its charter – first in 1847 in the crisis that coincided with the Great Irish Famine).
Indeed, the exception that proves the rule was the US Federal Reserve’s passivity which, it is generally accepted, lengthened and deepened the Great Depression of the 1930s. But that experience was not really taken into account in Maastricht Treaty planning.