I have decided to include an apico-labial nasal in my latest conlang. The sound is produced by voicing while simultaneously extending the tip of the tongue to touch the upper lip and thus prevent airflow out of the mouth while allowing airflow out of the nostrils.
I first encountered an apico-labial consonant in a playful speculative article by a scientist named I.J. Goode, who thought that the "pt" in ancient Egyptian might have been used to transcribe a voiceless apico-labial stop. However, I'm guessing that apico-labial consonants are found in natural languages, even if they are rare across languages.
I'm afraid that consulting this page, my usual go-to page for phonemic transcription, has only left me scratching my head, so any help here would be appreciated. http://westonruter.github.io/ipa-chart/keyboard/