Perhaps the right way to measure spillovers to the foreign country is to measure their welfare without the policy under question` and their welfare after the policy is implemented and response initiated.
So, for instance, a home country A at the zero lower bound may initiate Quantitative Easing (QE), and a foreign country B may respond by cutting interest rates to avoid capital inflows and exchange rate appreciation. The spillover effects of QE would be based on B’s welfare if QE was not undertaken versus B’s welfare after QE is initiated and it responds.