I hope the Black Heritage Project will write such a page for giving slaves a last name. I want to be sure not to offend anyone in the future, and neither do we want to be creating duplicate profiles based on different naming conventions. There may not be a clear cut method for naming our Black citizens who were enslaved, but to have an official page by the project will at least make all the relevant profiles consistent while reducing duplicates.
Another possibility could be to select a specific term for enslaved persons who have no last names. Just as Wikitree uses the term "Unknown" when the last name is not known, the term "Slave" or "Enslaved Person" could be used for all such profiles. It would make it easier to identify all such persons on Wikitree, and if such a term is included as the last name, it will catch a reader's attention that the biography may likely have additional details.
Also, on the edit page of any profile, when a parent is given, there are options for "non-biological," "uncertain," and others. Another option for this list could say, "Slave owner" and the slave owner could then be connected to the slave's profile but be treated as non-biological, the same as a step parent. Also, where the children of the slave owner are listed, a separate list will give the names of the slaves who are connected to the profile.