I think I read on Wookiepedia that one of the Legacy of the Force books had a someone visiting the One Sith from Legacy.
Yep, Alema Rar, a fallen and slightly crazy Jedi, visited the One Sith so the authors could specify their relation to what Lumiya was up to.
A certain One Sith with heterochromia (well, sort of) also made an appear in the final Fate of the Jedi
novel. Also Jacen specifically calls him the Dark Man he saw on the Throne of Balance, so his actions possibly stopped the One Sith from coming to Dominance within the next decade or so (based on Jacen seeing his daughter by the Dark Man's side) and delaying that conflict for a century.