Ultrafast photoinduced insulator-to-metal transitions in correlated materials are often mediated by lattice distortions, yet the role of interfacial lattice constraints in shaping nonequilibrium pathways remains largely unexplored. We use azimuth-resolved time-resolved X-ray diffraction to track orientation-dependent strain dynamics in granular V₂O₃ thin films on c-cut sapphire, where thermal-expansion mismatch imposes anisotropic interfacial strain. Across the thermal transition, the azimuthal profile of the (110)_H strain inverts curvature, providing direct evidence of partial clamping of the hexagonal basal-plane lattice (a_H,b_H). After photoexcitation of the antiferromagnetic insulating phase, the structural response remains clamp-limited: weakly constrained grain families reach the full basal-plane contraction characteristic of the metallic-like state, whereas strongly constrained families exhibit a strongly reduced distortion. Fluence-dependent measurements further disentangle transformed fraction from clamping-limited lattice distortion. Our results show that interfacial clamping acts as a static selector for ultrafast phase switching and provide a general route to quantify anisotropic strain dynamics in heterostructures.