I came across this:
"From the leather helmets formerly used by early Canadian armoured crewmen, with a pattern of stitching resembling a zipper."
Edward C. Russell (1980), Customs and Traditions of the Canadian Armed Forces, Deneau and Greenberg, Department of National Defence, ISBN 0888790279, p 65.
For the American use of the word, I came across this rather gory explanation of the origin:
"It is said to have been coined during the Korean war by frontline troops whom had run over enemy troops in jeeps.
The soldiers claimed that the tire tracks from the jeeps left a pattern resembling that of a closed zipper along the corpse."
I have read elsewhere, that rigored or frozen stiff bodies were actually used on muddy roads for traction.
I don't know how true it is, but Clint Eastwood used it, and he was speaking as a Korean War veteran.
This is from Wikipedia:
http://en.wikipedia.org/wiki/Zipperhead