My father has a photocopied record of the Mok surname that lists many of the early Moks. It lists the first Mok as being given the Mok name by one of the Tang Emperors in 851. Prior to that, the Mok lineage can be traced back to one of the Zhou Dynasty emperors, which subsequently became 'Gay' during the Han dynasty. I am not sure of the spelling, but that is the cantonese pronounciation. It is a descendant of the Gay that was sent down to Southern China by the Tang Emperor and given the Mok name (as well as some land).
Jennifer Mok, if you are still reading this, I wonder if we are looking at the same record?
I am looking into putting this document into an electronic fomat and hopefully putting a translation of the history of the Mok name on the web somewhere.
If anyone else has any information or insights into the Mok surname, please post.