The following will be no surprise for those of us who see lots of Greek inscriptions, but it may nevertheless be of some interest.
During the summer I had the opportunity to visit the New Acropolis museum in Athens. It is a great place, certainly when compared with the old one up on the Acropolis. For those of you who will go there sooner or later, there is an interesting inscription in the first major gallery (here, at the right hand side; sorry, I don't have a picture of the actual inscription). The inv. no. is EM 8123, is published as IG II² 2894, and is dated to the end of I AD.
The eight lines of this dedication in Greek to Apollo start off with mentioning the Latin name of the donor. His praenomen is Τιβεριος which is, naturally, abbreviated as τιβ. The interesting feature is that there is an overstroke over these three letters. There is no need to set these letters apart from the other text, as the praenomen occupies a line on its own (the first line within the enclosing wreath). I have seen the overstroke used for abbreviated names in more Greek inscriptions, always indicating Latin praenomina.
It is interesting to see that the overstroke for abbreviated names is a known feature in the first century and is part of the context in which nomina sacra came into existence. I think this phenomenon affects the balance of probabilities of two notions, but without settling the issue:
1) It is likely that the nomina sacra were 'designed' in a Graeco-Roman context.
2) The origin (or original idea) for the nomina sacra started with the name ιησους in the form ιης (with overstroke).