(1) Usually I would complain immediately about mixing up truthfulness, lying, and right-to-truth passages from the Catechism, but as Miller is explicitly focusing on the question of whether assuming a false identity is lying, so they all are actually directly relevant to the specific topic at hand. This makes the argument, in and of itself, one of the better ones I've seen.
(2) Miller says:
Aquinas approved of actual gestures of false signification stating: “A man may be deceived by what we say or do, because we do not declare our purpose or meaning to him.”
This is a somewhat baffling gloss. Precisely Aquinas's point in the discussion of ambushes is that these things do not involve false significations; these are cases in which we don't declare our meanings (we aren't signifying), and others are themselves responsible for the fact that they come to the wrong conclusions. The whole point is that we are concealing the truth, not that we are signifying what is false. And, in fact, Aquinas is quite clear that false signification with the intention to deceive is exactly what lying is, and always wrong -- his primary argument that lying is wrong is based on the issue of false signification.
(As a side note, Miller assumes throughout that the issue is intent to deceive; but in Catholic moral theology, intentio is a somewhat broader word than the English word 'intent'; it means the way you dispose yourself to act, which can include not just intent but a number of other things, like acquired habits. I don't think this affects her argument in any very serious way.)
(3) Catholics need to be careful with double effect when talking about lying. Lying is directly analogous to murder in these cases. When someone attacks you, this does not give you the right to try to kill them: that is murder, although under the circumstances much more excusable than murdering on a plan. You can see this quite clearly if you think about a scenario in which we find A and B, who are enemies. In this scenario, A is strolling along and suddenly is attacked by B; A suddenly realizes that now is his chance to destroy B once and for all, and he kills him. That is murder, plain and simple; the fact that B attacked A may make it harder to show that it was murder in a court of law, and it might, depending on the situation, make A's murder of B less serious than murdering on a plan; but defending yourself does not give a license for murderous intent, nor does it make intent that would be murderous suddenly non-murderous.
If you are attacked, however, it is entirely just to defend yourself from attack. In a life-or-death situation, the attack may well lead to killing your attacker. But the whole point of double effect is that for this to be just, you have to be out to defend yourself, not using the attack as an excuse to destroy your attacker.
The effect in cases of lying and deception, of course, is that the person in question is deceived; being deceived is the aspect of the case that directly corresponds to being killed in the self-defense case. It is not necessarily wrong to act in such a way that the other person is deceived; situations in which the other person is acting unjustly may indeed provide some fairly clear cases, although they do not do so automatically. But this is not a license to speak falsely with the intent to deceive. You must still be truthful; it's just that, as in the self-defense case, in which you have been, through another's injustice, placed in a paradoxical situation in which the effect is not one a just person would aim at, despite acting entirely justly, you have been placed in a situation in which being truthful happens to result in another's being deceived.
However, Miller (and this is a major strength of her argument) avoids the common error of trying to treat double effect as a blanket license here, and thus focuses narrowly on the question of false identity. And here she is entirely correct, as far as her argument goes. Having what we would call a false identity is not in and of itself an act of lying; and she is right that this is so even on a strict interpretation of Thomistic principles (given some things he says to Jerome, Augustine might be more skeptical). When Jesuit priests who were undercover in England during the reign of Elizabeth I went around under the name Mr. Smith (or what have you) to hide the fact that they were priests, this was not intrinsically wrong. To recognize this, it is not even actually necessary to get into the question of mental reservation, as Miller does and the Jesuits did, although this is certainly one way to go. If I call myself Mr. Smith, that's what I am called. If I, like the young Clive Staples Lewis, firmly resolved one day that everybody should call me Jack, Jack my name would be.
We get into more dangerous waters, however, if we are talking about not merely hiding my identity under an alias or pseudonym but impersonation. Miller's recognition of false identity as a form of self-defense is actually quite ingenious, but the argument necessarily comes with a very short tether. Just as in the attack, I must not be aiming to kill another human being but acting justly to both of us, so too in these cases of false identity one would have to not be aiming at false communication for the purposes of deception but exercising the virtue of truthfulness. And it is quite obvious that on this ground there are lots of things that you could not possibly do in order to create and maintain your false identity. I do not have sufficiently precise knowledge of how the videos were maintained to know whether the cover was maintained truthfully. But it is important regardless to understand just how small the room to maneuver provided by this argument is.
All in all, this is a good defense: it proportions its conclusion properly to its actual premises; while there are things I would change myself, it is laid out fairly well; it handles ethical analogies massively better than most discussions on this topic I have seen; it doesn't mangle double effect beyond recognition; and it doesn't treat the Catholic tradition of moral theology as a wax nose. I don't think it gets anyone all that far, but what it does get is properly earned, and I have no major complaints about it.