Epistemic Border Control as Proxy Regulation Under Partial Observability

Informal regulatory systems and institutional feedback controllers often attempt to govern targets — claim quality, truth-tracking, infrastructure burden, epistemic hygiene — under partial observability. When no real-time sensor on the target exists, or its signal-to-noise is dominated by a distinct proxy variable, the controller is structurally driven to substitute the proxy for the nominal target — strictly in the unobservable case, practically in the weakly-observable case as observability conditioning deteriorates. The rhetoric of regulation remains fixed on the nominal target while the actual controlled variable becomes the proxy. This paper isolates the substitution as a consequence of observability asymmetry, offers a finite-horizon observability-Gramian diagnostic for the substitution magnitude in the simulated regime, and operationalizes both via a single-agent Kalman-LQR simulation. The substitution does not require corrupt intent, gamed metrics, or slow drift: sincere regulators with a correctly-specified cost function and a correctly-specified plant model produce the same substitution under the same sensor geometry. The result is sibling to Paper 24's aggregation-layer masking by an algebraic argument — aggregation reduces variance as O(σ²/N) but does not rotate the observability subspace — and shares closed-loop substitution geometry with Paper 23's identifiability-masking apparatus. Informal discourse moderation is the worked case; the contribution is a control-geometry result that applies wherever a controller with a fixed sensor map confronts a target outside its observable subspace.