On a recent trip to Baltimore for a Bar Mitzvah, my uncle R. Hillel Tendler shared a story. Rav Moshe and Rav Hutner were once travelling together in a car, and they passed a street corner where there was someone asking those passing by to stop for a moment and put on tefillin. Rav Hutner pointed it out to Rav Moshe, perhaps lauding their efforts to reduce the numbers of קרקפתא דלא מנח תפילין. Rav Moshe responded that he didn't think it served any purpose at all. Rav Moshe explained that tefillin is an אות, meaning it is a way to indicate to Hashem that we are his subjects and willing to serve Him. One who wears tefillin and doesn't have this intent, has not fulfilled the mitzvah of tefillin.
Although those allowing the tefillin to be put on them were likely intending to do a mitzvah, Rav Moshe held that the mitzvah of tefillin is not merely the act of placing it on one's arm and head, but rather the intent to be subject to Hashem's mitzvos, therefore one who goes through the motions but doesn't recognize this, remains a קרקפתא דלא מנח תפילין. Perhaps he derived this from the fact that although in krias shema (va'eschanan) the Torah speaks about tying tefillin, in Parshas Bo it doesn't focus on the action at all. Rather the Torah says that the Tefillin function as an אות and as a זכרון, indicating that this is an essential component of the mitzvah. Based on this, I would suggest that the concept of not having a היסח הדעת in tefillin which the gemara Yoma 8 learns from the tzitz, is not a separate violation, rather an essential component to the proper fulfillment of the mitzvah of tefillin which demand a constant awareness of being subject to one's creator.
This story made me wonder about whether there is value in putting tefillin on people who are comatose in a hospital or hospice...
The gemara learns from the pasuk במתים חפשי that when a person dies they become חפשי from studying Torah and doing mitzvos. The implication of the gemara is that this is an advantage of those alive over those who are dead and can no longer perform mitzvos. Rav Moshe (Hearah # 74) asks that this doesn't seem to be a distinction between live vs. dead, because there are many live people as well who are incapacitated and unable to perform mitzvos? Rav Moshe explains that the point of the gemara is to contrast those who are dead and no longer have any connection with mitzvos to those who are alive. When a person is dead there is no longer any value in assisting them in doing mitzvos such as putting tzitzis and tefillin on them. So long as a person is alive, even if they are incapacitated and are אנוס in their inability to do the mitzvah, if one were to assist them by placing tefillin or tzitzis on them, they would be fulfilling a mitzvah. Another distinction is, whether it is permitted to daven in front of someone who can't perform mitzvos. Davening in the presence of a corpse for example would be לועג לרש because they no longer have any connection to mitzvos, but davening in the presence of one who is alive, just "tied up", is not in violation of לועג לרש since they are obligated in mitzvos.
It would seem that Rav Moshe when he recommends assisting those who are incapacitated in performing the mitzvah of tzitzis and tefillin, is speaking about one who is conscious and able to focus mentally on the mitzvah, but unable to do the act of putting on tefillin and tzitzis. However, if one is comatose and not mentally aware of the fact that they will be performing mitzvos, it would seem that there is absolutely no value in assisting such a person by putting tefillin and tzitzis on them. Leaving aside the chiddush in Rav Moshe's understanding about the special nature of the mitzvah of tefillin, there is a more fundamental problem that applies to tefillin and tzitzis equally. Even according to those opinions who hold that mitzvos don't require intent - מצות אין צריכות כוונה, that simply means that one doesn't need to intend to be fulfilling a mitzvah with this act. However, all agree that a מתעסק does not fulfill the mitzvah. One who has no awareness that he is ever doing a mitzvah, has a halachic status of a mis'aseik and is not considered to have fulfilled a mitzvah.
Therefore, for a sick patient in a hospital, when he has enough mental awareness that he will be fulfilling a mitzvah by the action he is doing, there would be a purpose in putting tzitzis on him. Regarding tefillin (assuming the story above is accurate), one must assess the patient to see whether they are capable of not just realizing they are doing a mitzvah, but recognizing an allegiance and obedience to Hashem through this mitzvah. But a patient who is not functioning mentally, such as cerebral death, or even a deep coma, there would be no purpose at all in putting tefillin or tzitzis on their body. It would seem logical though, that even a patient in a coma has a connection to mitzvos in the sense that they cannot be fed issurim. One cannot for example dress a comatose patient in shatnez clothing (unless they are in a state that they don't feel temperature and don't derive any הנאת חימום from the clothing, in which case there is no issur). And of course, if they reach a point of brain-stem death, they must be buried without delay.