Oh how embarrassing, putting my croaking next to an angel like Sinead!
No reference comes to mind. Adding extra verses on the outside of poems is found elsewhere, for example the final verses of the Ratana Sutta. These precedent shows that we should not see this as particularly unusual. In this case the issues are:
- It’s a different meter. the meter of the Metta Sutta is very distinctive and the last verse is quite different.
- The theme comes to a natural conclusion at the second last verse
- The commentary says that, having taught metta, the Buddha needed to introduce insight since having metta for all beings lies close to the view of a self. This reasoning smacks of a later time, and is not how metta is treated in the suttas. (Historically, after Ashoka, the concept of attā was contoverted by the Puggalavadins, and from this time the Theravadins became much more tense and strict in how they handled self/not-self.)
As always in text criticism, a single data point is by itself inconclusive. But multiple indications all pointing in the same direction make the conclusion fairly firm.