This document presents HySORB-FG, a normative technical standard that defines a structured framework for conducting temporary, reversible field demonstrations of simultaneous sulfur dioxide (SO₂) and carbon dioxide (CO₂) capture within existing wet flue gas desulfurization (FGD) systems. The standard is designed to enable disciplined, engineering-led evaluation of modified alkaline slurry chemistries under real plant operating conditions, without permanent modification to absorber vessels, ductwork, or major mechanical equipment. HySORB-FG establishes requirements and guidance covering system applicability, operating boundaries, slurry chemistry envelopes, phased test protocols, gas-, liquid-, and solid-phase monitoring, mass balance reconciliation, safety management, rollback criteria, and data reporting. A phase-gated testing logic is employed to ensure preservation of baseline SO₂ control performance prior to escalation toward evaluation of CO₂ uptake, aligning demonstration activities with utility risk management practices. The standard deliberately avoids prescribing specific chemistries, performance targets, or commercial outcomes. Instead, it provides a conservative, technology-agnostic framework intended to promote repeatable field testing, protect host facility operability and regulatory compliance, and support transparent interpretation of results. HySORB-FG is intended for voluntary use by utilities, industrial facility owners, engineering firms, technology developers, and researchers seeking to explore the incremental carbon capture potential of existing wet FGD assets while maintaining primary environmental control functions.