We consider measurements of exclusive rare semi-tauonic b -hadron decays, mediated by the b → s τ + τ - transition, at a future high-energy circular electron–positron collider (FCC- ee ). We argue that the high boosts of b -hadrons originating from on-shell Z boson decays allow for a full reconstruction of the decay kinematics in hadronic τ decay modes (up to discrete ambiguities). This, together with the potentially large statistics of Z → b b ¯ , opens the door for the experimental determination of τ polarizations in these rare b -hadron decays. In the light of the current experimental situation on lepton flavor universality in rare semileptonic B decays, we discuss the complementary short-distance physics information carried by the τ polarizations and suggest suitable theoretically clean observables in the form of single- and double- τ polarization asymmetries.