I'm sorry to be confusing; I see cannibalism as orthogonal to death; one can amputate one's own leg and feed it to one's friends, or one can die of natural causes and permit others to consume the remains. In the grandparent, I wasn't considering dying being a part of the process of cannibalism.

As to dying for the purpose of being consumed, I don't think sane humans can consent to that, but other intelligences could, as long as they felt that the cost of dying was not high (i.e., they are confident that their goals will be accomplished regardless of their death). This is unlikely, but at least possible in my conception.

Assisted suicide clinics exist legally in Switzerland, and they require large amounts of proof that wanting to die is sane under the circumstances (usually a sharp decrease in quality of life because of some chronic injury or illness, with no cure and that is slated to get worse over time). I'm pretty sure they don't accept people suffering from ennui.

My point being, I think a strong enough version of consent already exists and is in use.

"but also maintained that position through a year of competent therapy and antidepressants."

Having been on antidepressants for a year, I'd point out I'd be significantly more inclined to let someone cannibalize me if I was on them. Neurochemistry is fickle and individual, and those things do not always do what it says on the label...