There are DeNeal message boards at www.Ancestry.com and www.genealogy.com. There is a recent message from Jennifer DeNeale, who indicates that she has done extensive research on the James DeNeale name. Also, Karen Ward was helpful as well. The Fairfax county courthouse would be a good place to start your research and the on-line University of Va library is excellent.
Let me know if you find any reference to DeNeal slaves in your research.