An apparatus includes a housing, a platform, a buttress assembly, at least one retainer member, and a lockout alignment feature. A portion of the platform is exposed in a gap defined by the housing. The buttress assembly is positioned on the platform. The buttress assembly is exposed in the gap defined by the housing. The at least one retainer member is configured to selectively retain the buttress assembly against the platform. The at least one retainer member is further configured to release the buttress assembly from the platform in response to a clamping action of an end effector positioned in the gap defined by the housing. The lockout alignment feature is configured to prevent the at least one retainer member from releasing the buttress assembly form the platform unless the end effector is located at a predetermined position in relation to the buttress assembly during clamping of the end effector.