Looking for something other than plain or Black Oxide coated M8-M10s Class 12.9 that are available up to 110-120mm long.

These will be used for Alternator mounting and it's not direct bolt-on so highest strength possible is being applied here.

why 12.9 ?

strongest stuff I know of that doesnt rust are studbolts with rolled tread (vs cut) used in the chemical industry, and I'd recommend a tough steel rather than a hard steel (but I'm no exert on this field)

I'd contact a company like this http://www.fluidsealingproducts.com/stud_bolts.html

if you havent found anything call fastenall

I did. They sell Zinc plated bolts. Not as corrosion resistant as i'd like but better than nothing.