Well... let's Trope this.
A Posthumous Character is one that is a prominent character, but happens to be dead for the majority of the movie. A character in a movie (as opposed to an extra/human prop) is part of the plot. To paraphrase from the Trope page...
In Children of Men, the reason the child Dylan was even born was to die and provide drama and motivation for the events that occurred during the movie.
I suppose it is also possible that Dylan is a Death by Origin Story... as Dylan's death was part of what made our heroes who they were during the movie.
Now... I deleted this at first, because I recognized the fact that this wasn't what you were asking. So, I started digging. And although Tropes doesn't specifically deal with the Unseen Character, they do deal with breaking up the concept into several smaller categories. This still doesn't answer your question.
@Crow's answer actually (and I hope he brings his answer back) is mostly going to be correct for this. There doesn't appear to be a small subset of names that gets used to cover the unseen characters in stories and movies. Actually, these unseen characters all appear to be very specific people who are designed to be memorable. Because we do not see them, we have to work with things like their unique names or habits to make them more memorable. Thus... the names of these characters tend to not be all that common.
That said, when these unseen but important characters are given a name to reference them, the most common ones are those given to corpses and unknown suspects in crime-based entertainment. That's why Crow's answer would technically be correct.
It would be interesting to use the sites with the lists of the most popular unseen characters, as well as what information is in IMDB to compile a list of all the documented unseen characters... to see what pattern might emerge. It doesn't appear that anyone has done this yet. I would surmise that you might find a name like Joe, or John... because it is relatively common... to have multiple references. I do not think that you would find a a small number of names used over and over again (unless there was another reason for the use of those names like I detailed above).