Dilute-carbon defect-chemistry entry for the RE-Free Permanent Magnet Leaderboard: 4x4x2 L1_0 tau-MnAl supercell (a=2.792 A, c=3.584 A, literature-parametrized tau phase) with one C interstitial at the octahedral (1/2,1/2,0)-type site (4 Mn + 2 Al coordination), Mn32Al32C = 1.5 at% C, matching the ~1-2 at% C that stabilizes the metastable tau phase in MnAl-C magnets. Companion to the no-C control supercell (file 54f0b572). Built with pymatgen; validated: P4/mmm preserved, min pair distance 1.79 A, all sites ordered.