The time-dependent asymmetry in $B^0 \to K_{\rm res} \gamma \to \pi^+ \pi^- K^0_{\scriptscriptstyle S} \gamma$ is sensitive to the photon polarisation in the quark level process . While this polarisation is predominantly left-handed in the standard model, it could be modified by the existence of new physics contributions that may possess different properties. In this paper, we derive the violation formulae for $B^0 \to K_{\rm res} \gamma \to \pi^+ \pi^- K^0_{\scriptscriptstyle S} \gamma$ including the most dominant intermediate states. We propose a new observable that could be measured in a time-dependent amplitude analysis of decays, providing a stringent contraint on the photon polarisation. We discuss the future prospects for obtaining such constraints from measurements at Belle II and LHCb.

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