I've found the earliest Civil War use, which appears to be this (originally posted to the American Dialect Society list in 2000):
Eye of the Storm: A Civil War Odyssey
written and illustrated by Private Robert Knox Sneden
Edited by Charles F. Bryan, Jr. and Nelson D. Lankford
hardcover, 329 pages, $37.50
The Free Press, 2000
See the article in today's Newsday, 8 January 2000, part 2, pg. B3, col.2, "Civil War 'Treasure.'" The article has a nice illustration of the "deadline" that's on page 221, from April 8-20, 1864.
8 May 1910, New York Times, pg. 1:
DIES OF CANCER
Made Fulton Street a "Dead Line" to
Crooks and Introduced His Own
Methods of Controlling Crime.
On the day of his appointment he established an office in Wall Street, and announced a "dead line" at Fulton Street, promising immediate arrest for any crook that ventured below it. In Wall Street Byrnes made many friends, and through information given him by the rich men whom he protected by his "dead line" policy he accumulated a fortune.
These articles describe the Fulton Street "dead line" (1879 or 1880) and the Fourteenth Street "dead line" (some time before 1910):
3 August 1927, Nevada State Journal, "New York Day By Day" by O. O. McIntyre, pg. 4, col. 5:
Maiden Lane houses the bulk of New York's fine unsold jewels. The few crooks who have ventured below the dead line at Fulton street call it "the street of suspicion." Loiterers are not welcomed and many eyes are on constant guard. When a pedestrian stops too long he will find someone a short space away ready for an emergency. Every employe of responsibility had a permit to carry a pistol. So carefully is it protected, it is said, that in two minutes the thoroughfare can stop all arrivals or departures.
Inspector Byrnes established the crooks' "dead line" at Fulton street. It was done to protect the vaults of hoarded gold in Wall street. The crook who violated this unwritten law became a marked man. He was trailed every moment until he was caught in some crime or found the espionage so annoying he left the city. Byrnes' order was really a test if the "honor among thieves" theory. If one strayed over the line he received the contempt of his guild along with police vengeance.
4 December 1910, New York Times, pg. SM3:
And knowing the people of his district as he does, no one is more familiar with the reign of lawlessness that has run on practically unchecked below the Dead Line. There is seldom a month that some brutal killing is not recorded in that section of the underworld which lies to the southward of Fourteenth Street.
For more on the "dead line" and the "Dead Man's Curve," see this:
7 May 1893, New York (NYTimes, pg. 16:
CABLE CARS ON BROADWAY.
Superintendent Newell Says They Will Soon Be in Use.
31 January 1897, New York (NY) Times, pg. 14:
The Deadly Curve.
2 April 1897, New York (NY) Times, pg. 12:
THE UNION SQUARE CURVE.
He said that the curve at Union Square has been a dead line to business below it, particularly to carriage trade.