I don't believe Pratchett signed up for cryonics, but if he had, what good argument could there be for not letting him commit assisted suicide in the way that best prepared his body for cryopreservation, followed by immediate cryopreservation?
If medical science admittedly can't do anything for these people other than offering palliative care, it seems like from a sanctity of human life perspective, cryopreservation under optimal conditions at least offers a chance to preserve the life that assisted suicide opponents hold to be so sacred. Maybe rather than repealing laws against assisted suicide, we could work towards getting exemptions for cryopreservation?
To the best of my knowledge, countries which allow assisted suicide frown upon cryopreservation, and vice-versa. Many places also require autopsies for suicides; I'm not sure whether that applies to the assisted ones.
http://blastr.com/2011/06/terry-pratchett-begins-fo.php
Another great mind that will be needlessly lost forever. :(