This work investigates CPT and Lorentz breakdown in neutral-meson interferometry within the nonminimal Standard-Model Extension. Employing methods of chiral perturbation theory for mass dimension d = 5, a connection between the quark-level and the meson effective pseudoscalar Lagrangian is established. The resulting phenomenological signatures in neutral-meson oscillations with particular focus on the detector geometry are explored. This analysis permits the interpretation of such CPT tests within the d = 5 quark sector of the Standard-Model Extension. Existing data is used to extract first constraints on two coefficients and to estimate achievable sensitivity gains over published bounds of up to 18 orders of magnitude.