I did find a list of early slaveholders named Houston here:

http://freepages.genealogy.rootsweb.com/~ajac/surnamesh.htm

You could also try the historical society here:

http://www.schistory.org/searchcatalogue.htm

I found a John Houston, mulatto,bricklayer, listed in Beaufort SC in the 1860 census, living in St Helena Parish.

There are 4 mulatto and black John Houston names in SC in the same general area in 1870; one white, but he was living in Charleston:

John, age 31, a builder, and Catherine, age 22, Houston, daughter Mary Eliza. Eliza Fell, 14 black, servant; Sarah Braughten, black, age 19, a cooke, Anna Braughten, age 10, mulatto. Eliza, Sarah and Ann are free blacks.

In 1860 in Edgefield Cty, Hamburg, SC there is the following:

JA Houston, negro trader, wife Amy, children Mary, Robert A., Amelia, Alice. Also an Eliz. Knight, age 60.

John Houston, age 8, black, farm laborer on the farm of a John Jenkins in Colleton Cty, Blake Twp, also 1860. this John Houston is a free black.