I think the main advantage with starting moe with a story is that it supplies the rules of the 'universe' where the rest of the work will develop. This can be very helpful if you want to explore a universe with rules different from our own. For example, one where animals speak or magic is real.
The story creates the primary text, as it where. These are the non-negotiated 'givens'. For example, the primary text for Red Riding Hood, is that wolves can talk and question the girl and that Grandma's can be swallowed and live inside wolves' bellies. But this wouldn't be true in the Neil Gaiman's The Wolves in the Walls'.
The secondary text is the one negotiated with the children. If you are using moe then one of the children's points of view needs to be as experts. They don't have be 'a company' by this I mean a business, but they do need to be a team of experts, with the responsibilities that go with expertise. For example, if a class of young children are exploring the structure and characters of fairy tales, through Red Riding Hood, then it might make sense for them to be a team of problem solvers, who are commissioned by the village to protect and educate their children from the dangerous wolves who inhabit the forest. Or (for older children) exploring childhood fears and prejudice (through the The Wolves in the Walls) it might make sense for them to work as an animal welfare team with the task of relocating a pack of wolves (or maybe foxes) who have been driven into the city by hunger and hunting.
The third (or tertiary) text is the level that the children invent for themselves, either individually or with one or two others, but which is never agreed to be part of the larger 'story'. For example, they might draw pictures of what they think the wolf (wolves) look like, or imagine the sound they make, their breathing, their howling; they might invent the stories of what drove the wolves to act so recklessly and to take such risks. The difference between the tertiary and the secondary text is that although these thoughts and ideas can be shared with the class as a whole they are are never agreed as being the they only way it is, just possible ways. This is important because it allows children the space to see that there are many ways to see and to interpret the world.
I find the best way to start an moe inquiry when using a story is to ask the children to image something that was not on the page. For example, in Red Riding Hood, I might ask, where does the wolf live or what kinds of things do people in the village say about the wolves in the forest or what jobs were the girl's parents doing that meant they were so busy they couldn't go with her into the dangerous forest? In this way I can first build the context and then introduce someone from the village who will ask the question that will get us the expert team: "We're so worried about these wolves. We know they are hungry and desperate and we don't know how to protect our children." Of course, later on, the problem solvers might also get a visit from the wolves pack: "We're hungry and the human's hunt us, we need meat to feed our young, what can we do?"
I hope this helps. Tim

When I was new to the approach, using a book was the best way I could find to start. I think I ended up doing 2 terms that started with Owl Babies (KS1) and went through rescuing fallen owls, to setting up a hedgehog rescue and information service. Mantle is all about stories, the sort of stories that matter and ones that can be related to. Words of warning, though, less is more and slow it down! It may not be so much about finding out what happens in the book but following the questions (lines of enquiry) that come from it.
I would visualise it with the book as a thin layer of cheese at the heart of a big doorstep sandwich (Mantle being the bread). Dene