"That said, we're getting positive reactions from people close to our target audience, such as high school teachers, people teaching others how to make their first app, etc."
I bet. But this is part of the problem with many of Mozilla's products targeting the education space (Open Badges, I'm looking at you). Teachers, administrators, bureaucrats think they're the cat's meow. But (of course) they want nothing to do with the implementation details. But they do expect them to be usable.
"At the highest level, we're exploring whether it's possible to make a tool that lets non-devs (_not_ you folks!) who currently see their phones as a pure engine of consumption, as a place where they can create something fun or useful."
That's a fantastic idea. So are open badges. But "us folks" are the ones that (whether we work in edu IT, at an edtech company, or are fellow travellers) are expected to make these half-baked great ideas work, reliably, in the real world, for educators and students.
I'm not trying to rant. Mozilla has great ideas for education.
But the poor (maybe I should be more generous and say incomplete) implementation of so many of these great ideas (Persona, Open Badges, and now Appmaker) is maddening when they're presented in a way that "not us folks" expect them to be available for production implementation.