It's all been done before:
paintedcows.com, the advertising and marketing resource, launched its website in November 1999 with an advertising world's first: an organic billboard.
The idea for advertising on live animals was born on a long train trip across America's heartland. Site co-founders, Shelley Delayne and Scott Karlson, speculated that all of those cows grazing along highways and byways are uniquely qualified as eye-catching advertising space.
The partners decided that this concept was to be the cornerstone of their efforts and rented a cow named Specs to become the world's first Ad Cow.
see:
http://paintedcows.com/specs.html