The practice of playing a melody off beat against a stridant beat and using it as a basis for rhythmic improvisation was referred to as "ragging" and seems to have been a common practice among minstrel show banjo pickers and other instrumentalists from the the end of the civil war on--the device is pretty much the basis of all the "American" music forms, from ragtime, jazz, and blues, to bluegrass, travis picking, funk, rock and roll, and so on--it also commonly used in African and Arabic music--

Seems to me that I ran across a copy of an audition notice for minstrel banjo pickers from 1880 or there abouts to the effect that prospective players had to play a two minute improvised rag based on a melody that would be given to them at the time of the audition--

Interestingly, the elements of rags and Indian ragas are fairly analogous==