Above: A statue of the original 12th Man, E. King Gill, in front of Texas A&M's Kyle Field
Don't let it stop you from painting your head blue and white, but the "12th Man" didn't originate in Seattle.
The 12th Man tradition dates back all the way to 1922 at Texas A&M:
"The tradition of the Twelfth Man was born on the second of January 1922, when an underdog Aggie team was playing Centre College, then the nation's top ranked team. As the hard fought game wore on, and the Aggies dug deeply into their limited reserves, Coach Dana X. Bible remembered a squad man who was not in uniform. He had been up in the press box helping reporters identify players. His name was E. King Gill, and was a former football player who was only playing basketball. Gill was called from the stands, suited up, and stood ready throughout the rest of the game, which A&M finally won 22-14."
Texas A&M still owns the trademark, licensing it to the Seahawks for a sum of $5000 a year after Seattle paid an initial $100,000 upon signing the contract in 2006.
Good Bull Hunting has an interview with Shane Hickley, the Assistant Vice President of Business Development at Texas A&M, about the 12th Man licensing deal. It includes a tidbit about the university sending out around 200 cease and desist letters over the past two years to people violating the trademark license.