I finally saw the pink apron make an appearance, but it wasn't a CG or even a full scene. Tocchan's apron was getting old, so Haato offered his as a replacement. Not long after, foreplay started and Tocchan was wearing the apron but that's it. No other comments on it or anything, just that he's wearing it. Bit disappointing that there wasn't a scene for it but oh well.
Meanwhile I'm just realizing that I've had the Gakuran/teacher-student scene the whole time and could have told you that was the one you were missing (and how to get it) if I'd been paying attention. oops