Cosmic Bell tests employ photons from distant astronomical sources to determinemeasurement settings, thereby constraining local hidden-variable models by extending thecausal origin of experimental choices deep into the past. These experiments implicitlyassume that photons arriving from cosmological distances constitute an unbiased,transport-neutral sample of emitted states. The Spacetime Phase-Structure (SP3)framework introduces a distinct possibility: that long-baseline propagation throughstructured vacuum-phase coherence domains may selectively condition transport modes,introducing subtle, path-dependent statistical skews without altering Bell-inequalityviolations themselves.Here we propose a Bell-style experimental variant designed not to test locality or realism,but to probe transport-level neutrality of light propagation. The protocol comparespolarization statistics and randomness metrics derived from cosmic photons spanningcontrolled age and propagation-distress cohorts, while holding laboratory entanglement,detection hardware, and Bell analysis fixed. The experiment is executable with existingquantum-optical and astronomical instrumentation and yields falsifiable outcomes thateither reveal or constrain SP3-style coherence-filtering effects in spacetime transport.