"...a map of 'Truce Terms' (Fainites, Kings, Pax, etc) showing the range of terms and where they were used. Does anyone know of anything similar for 'endearment terms'"

Now the people who could answer that would be The National Centre for English Cultural Tradition.

I found that link while looking for the "Survey of English Dialects", which is the topic of this page from a BBC site - with a link to a fascinating sound file of an interview, with some recordings of dialects that'd defeat most listeners to understand.