In Rav Yitzchak Ginsburg's Introduction to the Kabbalah of the Ariz"l, (pp. 169-171) he brings down the idea (a favorite of the Baal HaTanya's) that the willingness to sacrifice one's life for the sake of HaShem's name is something that resides at the root of our souls, in a part called the Yechidah. This highest part of the soul is never actually clothed within us, rather it surrounds us from above. It is well beyond our mental faculties, above even our decision-making process.
In light of this, there isn't really a question as to what happens if someone dies in HaShem's name without being given the choice. In essence, we don't really ever have a choice in the matter. Every death for the sake of HaShem's name is holy because it is a revelation of the deepest essence of a Jew.
To me, on some level this would seem to be troubling. A case of no free will, how can we see something as honorable or laudable if there was no choice involved? One obvious answer is that HaShem never really lets any of us choose how and when we die---even if we think it is within our own power. The Mishnah tells us in Pirkei Avoth:
על כרחך אתה חי. ועל כרחך אתה מת. ועל כרחך אתה עתיד ליתן דין וחשבון - against your will you live, and against your will you die, and against your will you will stand trial.
So what is important? What you did all the time between life and death. That's what you are going to stand trial for. That's the more technical answer.
The more spiritual answer is as follows: What's the goal of being a Jew? To bear witness to HaShem as Creator, Master, and King of the world. Choosing right over wrong is only a means to this end. When we choose right over wrong we are bearing witness to the fact that HaShem created us and gave us the commandments to perform along with free will to not perform them.
[In a way an even higher level of this revelation is when we actually break the commandments, proving that we don't have to keep them, and then we do teshuvah and keep those very same commandments. Then we've removed all doubt that HaShem gave us free will, and yet we serve Him because His ways are Emet, truth.]
Through our actions we are revealing our pedigree, that in fact, we are of divine origin. HaShem did create the world and us in it.
When we see the actions of a Jew as the revellation of the underlying Jewish Neshamah, then we can see too how a Jewish death in and of itself is special because it reveals the essence of the Jewish soul that just left this world. The Baal HaTanya even says that a person who dies a Jew, was always a Jew, and a person who dies a non-Jew was never a Jew. (This is a tremendous chidush, because without this definition we always understood than anyone born a Jew is always a Jew. Not so according to the Baal HaTanya, the person's end reveals his true beginning, his true essence. נעוץ תחילתן בסופן)
So, no matter whether a Jew had the choice to die as a Jew or not, the fact that they died for the sake of HaShem's name reveals their spiritual essence, that they are and always were a Jew, that HaShem is the Creator, Master, and King of the world.
[There are many opinions of what is the point of being a Jew. Each of them has their own subtle points and differences, I take my cue from Avraham Avinu. What did he do differently from Noah? He actively engaged the world to glorify HaShem's name.]