The main point of punishment is to deter crime, rehabilitation is secondary. One to think of this using TDT is that the point of punishment is to acausally prevent crime. I discussed this in a slightly different context here.

As to whether a criminal should be allowed to sign up for cryonics, that depends on what the appropriate level of punishment for his crime is. After all, we assign some criminals 10 years in jail, others life, and execute still others.

As the question of mental illness, as Yvain points out in Diseased thinking: dissolving questions about disease.

Instead of continuing the fruitless "disease" argument, we should address these questions directly. Taking a determinist consequentialist position allows us to do so more effectively. We should blame and stigmatize people for conditions where blame and stigma are the most useful methods for curing or preventing the condition, and we should allow patients to seek treatment whenever it is available and effective.