From a David Weber post to Baen's Bar Snerker's Only dated August 13, 2009:

Timothy Mears and DNA access

    Ah, guys, I wasn't going to mention this just yet, but since the point's been brought up, it's been emphasized from the beginning by the Bad Guys that there are significant difficulties in selecting "suitable vehicles" for these types of assassinations. Some people seem to be assuming that the difficulty lies in something about the genetic profile of the chosen programee, but I don't believe I've ever said anything of the sort. If I have said so, or if I even seem to have implied that, it was unintentional, because that isn't the actual problem, at all. The problem is selecting someone in a position to carry out the desired attacks/mission to whose genetic profile one already has access.

    This access can be acquired in several ways, but it isn't exactly a nontrivial exercise to get one's hands on a genetic sample of someone serving in a hostile navy 700 or a thousand light-years away from one's Evil Laboratory. It may or may not ever be completely specified in one of the books, but the nanotech used on Timothy Mears had to be very carefully prepared ahead of time, based on a DNA sample acquired (by so far unspecified means, although I've got three different scenarios in mind), and then delivered to an agent already in place on Manticore (the charming taxicab driver) for ultimate delivery to him. Please note that we never see the taxi driver again, thus -- quite unintentionally, I am certain -- leaving the door open for all sorts of speculation. Was she actually a field agent of Alignment Security who knew all about the Onion? Was she an agent for the official government of the Mesa System, who had no idea she was working for the Alignment? Was she an agent for Manpower, who, once again, would have no idea she was working for the Alignment but could probably see all sorts of good reasons for Manpower to absolutely hate one Honor Harrington? Was she a disposable local stringer, selected for her mission, paid for it, and then, like the computer hacker involved in setting up the Webster assassination "payoff" paper trail, simply removed? I won't say the possibilities are endless, but they're certainly… extensive.