Would it be culturally inappropriate for me (a caucasian ameriken) to say "neg" while in Haiti? I see it in phrases "neg di san fè", and even see "granneg" in the New Testament.

No. Not at all.
You can even call yourself yon nèg blan (a white man :)
The word 'nèg' in Haiti translates man, guy, partner, boyfriend,  friend in English

See the links to these posts: nèg is not the N word.

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words