Doug I have got a copy of Manuscript, Society and Belief somewhere, but I don't know where among my books. I cannot say for sure exactly what Roberts says. In general he argues for Christian innovation in scribal practices including the nomina sacra. And like Hurtado, he argues that Jesus was the first nomen sacrum, based on Christian gematria and the word "life". But I would be surprised if he argued there was no connection with the Tetragrammaton and its treatment. In fact I am pretty sure he allows for some connection, as Hurtado and others continue to do. The nomina sacra occur within a context in which reverence for the divine name is important, although the direct relationship between the two is difficult to pin down on the available evidence.
jhine if we only had access to copies of the LXX from the third century onwards there would be no direct evidence for the divine name in the early LXX either. Somehow Christians managed to remove the divine name from the textual tradition of the LXX at an early stage. So the argument is quite straight forward: if Christians succeeded in removing the divine name from the LXX the same could have happened in the textual transmission of the New Testament. So it is certainly feasible that this happened. That having been conceded, it becomes a matter of looking at the evidence for and against. I would argue that the strongest evidence in favour of the divine name in the original NT is the many passages that make much more sense if it used the divine name. For example the passage in Acts that specifically talks about the divine name in chapter 15, plus the recurring quotation of the phrase "the Lord said to my lord" from the Psalm, which struggles to even make sense without reference to the divine name.