I have.

Before the arrival of the superhappies, my preferred strategy would have been to explain to the babyeaters that we had made some initial experiments in baby-eating but been blinded to the underlying goodness of the act for reasons of economic expediency. I would then demand that the babyeaters - all of them, on all their ships and worlds - hand over all the children from the current generation who would otherwise have been eaten, so that all of us humans could figure out how to do it properly as soon as possible. If they balk at the logistics of such a sudden, massive tributary payment, I would point out the horrible possibility that entire worlds - billions of sapients - might otherwise carry on for years in ignorance of the proper practice of baby-eating; if that doesn't work, I'll politely remind them that we've got overwhelming military superiority and as such they are in no position to dictate terms. Nobody starves, because baby-eating has become metabolically redundant, and the children thus abducted are raised in a non-baby-eating culture. Upon returning, they could convince their parents - by sheer weight of numbers - that this whole baby-eating thing was just an honest mistake.

Once the superhappies show up, that plan goes out the window. Since we have, in any sane game-theoretical sense, established peaceful relations with the babyeaters, shared most of our military secrets with them in fact, an attack on them could be interpreted as an attack on us, and should be discouraged accordingly. Anyone with a competent lawyer would know better than to identify themselves as an authorized representative of all of Humanity, and given a moments' consideration, remember how people have responded to "feelings greater than love" in the past.

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...