Interesting speculation, Coberly, but 'cowboy' goes way back. In the UK it literally means a boy who tends cows (Oxford English Dictionary) and is in print from 1725.
The name 'cowboy' was applied to some barbarous tory partisans during the Revolutionary War (in print in 1775).
In 1849, Jenkins, in "Hist. of the War U. S. and Mexico," wrote of Texas cow-boys attacking Mexican 'rancheros' who crossed the Rio Grande, perhaps the first reference in print to the 'cowboy' as we know him in America (OED), thus indicating earlier use of the term.

In Spanish used in North America, vaquero (cowman) is the word for a cowboy.
Donald Gilbert y Chavez, in his excellent "Cowboys - Vaqueros, Origins of the First American Cowboys," (Univ. New Mexico Press, and internet- http://www.unm.edu/~gabbriel/index.html )- says "Cowboy - a transliteration of the Spanish word vaquero (cowman) into the English cowboy, widely applied term used to refer to men who tended livestock..., also cowhand, cowpoke, or cowpuncher." He doesn't believe that American usage of 'cowboy' is descended from English and British Isles usage.