An apparatus includes a housing, a platform, a buttress assembly and a lockout feature. The housing defines a gap that is configured to receive a portion of an end effector of a surgical stapler. A portion of the platform is exposed in the 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 lockout feature is configured to enable an end effector with an unspent staple cartridge to clamp onto the platform and thereby engage the first buttress assembly. The lockout feature is further configured to prevent an end effector with a spent staple cartridge from clamping onto the platform to thereby engage the first buttress assembly.