Hey I like the theories that everyone is coming up with here. I think that I might have some more evidence to support the theory of the Ceremony involving someone eating Bree. According to the LGPedia, Thelema was created by Aleister Crowley as a religion with some Ancient Greek references. A great example is the name Thelema translates to 'will' in Ancient Greek.
You might be wondering how this relates to Cannibalism. Well, thanks to the wonderful invention called wikipedia, I looked up cannibalism to try to find something that references Thelema with cannibalism. Well, I did find this:
A number of stories in Greek mythology involve cannibalism, in particular cannibalism of close family members, for example the stories of Thyestes, Tereus and especially Cronus, who was Saturn in the Roman pantheon. The story of Tantalus also parallels this. These mythologies inspired Shakespeare's cannibalism scene in Titus Andronicus.
So, if certain Greek myths referenced Cannibalism and Thelema is Greek for 'Will', then maybe there can be more Ancient Greek influences in the religion. This might be the reason why eating Bree might sound like a great idea for the Order.