I was working on culture specific unit skins starting with the industrial to modern era. Although I have made progress in a fairly quick method of changing the skins, no work on that project has been done for a few weeks but I'm sure I'll take it up again shortly.
The process is actually simple with the most difficult part being to adjust the brightness/contrast of the skin+gloss map to match a corresponding game unit. Perhaps I should post a little tutorial on how to do it. (It would probably answer alot of questions.)