Incredibly late to the party here, but I just wanted to point out that while I can't speak for Greek, I can tell you that the K-sound has only one letter in Russian: both C and K get transliterated as K. So Konstantine could be both Greek or Russian.

Another possible idea is that "real" Barrayaran Greeks have kept large chunks of their culture as well as their language, but those who aspire to Imperial Service, in whatever form, take great pains to appear smooth and urban, and ditch as much of their home accent and attributes as they can stand.