We consider measurements of exclusive rare semi-tauonic b-hadron decays, mediated by the Formula omitted 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 Formula omitted decay modes (up to discrete ambiguities). This, together with the potentially large statistics of Formula omitted, opens the door for the experimental determination of Formula omitted 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 Formula omitted polarizations and suggest suitable theoretically clean observables in the form of single- and double- Formula omitted polarization asymmetries.