Let me narrow the discussion down to one of the more widely-known Nahuatl names: Nezahualcoyotl.  In Spanish, this is usually but not always written: Nezahualcóyotl.  The Nahuatl Wikipedia does have an entry for him: nah:Nezāhualcōyotl (with horizontal accents over the vowels).  However, there seem to be very few English Google entries which use accent marks.  The Encyclopedia Britannica article has no accents, fr'instance.  I would suggest that we should either use the two vertical accents (as per Nahuatl Wikipedia) or none at all.  I completely dislike the idea of following the Spanish example since Nezahualcoyotl was not Spanish in any way and this is not a Spanish language dictionary.