Jack Campin, with regard to your question: "where were the earliest known venues for proto-minstrel Black American dance? It must have come from somewhere before it became showbiz. Where were "plantation dances" done?"

My response is that proto-minstrel Black American dance was done where ever African Americans congregated, in rural areas {plantations and farms} and urban areas, including places specifically for African American socializing such as New Orleans's Congo Square.

For instance, read this excerpt from an online article about New Orleans- Birthplace of Jazz:

"...the birth of jazz is still something of a mystery. 100 years ago, at the time jazz emerged as a distinct musical genre, there was no recorded music, nor was there radio. No documentation exists that can in some way pinpoint the birth of jazz, since the music was played by ear. What is known is that the Jazz era began around the time the Ragtime era ended, and early New Orleans jazz musicians actually spoke about their music as a local flavor of ragtime.
New Orleans, because of its distinctive cultural life, history, and geography, was able to provide the crucial added ingredients to ragtime that turned it into jazz. The Port of New Orleans was one of the most important terminals in the New World, and as such facilitated a great exchange of cultures as well as commerce. The city's strategic position on the Mississippi was of great interest to European colonialists and traders. And, as the Civil War had ended only thirty years before, the Delta slave culture was still lively.

In New Orleans's Congo Square, a grassy plain where the city fathers had permitted slaves to fraternize for a few hours on Sundays, revellers performed African dances accompanied by music that featured drumming and stringed instruments. American Indians, slaves, and free people of color conducted business, socialized, and entertained themselves. The square is listed on the National Register of Historic Places."
-snip-

Here's more detailed information about Congo Square from http://www.jass.com/congo.html :

"It was in the Nineteenth Century in Congo Square in New Orleans that observers heard the beat of the bamboulas, the wail of the banzas and saw the multitude of African dances that had survived through the years. This square located across Rampart Street on the back side of the French Quarter was in use as a gathering place for the residents of New Orleans almost since the city began. It had been an area outside of the fortified walls of the original city where Native Americans and later slaves had sold their wares in an open market by the Bayou St. John, the major avenue for transportation of goods into the city.

Town's folk would gather around the square on Sunday afternoons to witness what went on inside the square. In 1819, a visitor to the city, Benjamin Latrobe wrote about the celebrations in his journal. He was amazed at the sight of five or six hundred unsupervised slaves that had assembled for dancing. He described them as ornamented with a number of tails of the smaller wild beasts, with fringes, ribbons, little bells, and shells and balls, jimgling and flirting about the performers legs and arms. The women, one onlooker reported wore, each according to her means, the newest fashions in silk, gauze, muslin, percale dresses. And the males covered themselves in oriental and Indian dress and covered themselves only with a sash of the same sort wrapped around the body.. except for that they go naked.

One witness from the time pointed out that several clusters of onlookers, musicians, and dancers represented tribal groupings with each nation taking their place in different parts of the square. In addition to drums, gourds, banjo-like instruments and quillpipes made from reeds strung together like panpipes, marimbas and european instuments like the violin, tamborines and triangles were also used".