subject matter

Subject matter consists of the facts observed, recalled, read, and talked about, and the ideas suggested, in course of a development of a situation having a purpose.

I think that putting up a tent with other people would fit Dewey's definition perfectly.

Can't really add much to your citations and observations since they seem to truly grasp the import of what Dewey observed (then) and also the relevance to things as they are generally done now.

Thinking about these things and trying them out (in the classroom etc as you describe) is exactly what Dewey had in mind.   He did not intend his thoughts and observations to be used like recipes in a cookbook.  Rather, it's the ideas and principles he articulates that need to be experimentally put into current practice.  They need to be recreated or reconstructed into our own ways of doing and thinking.

I think most of what Dewey writes about in these core chapters (11-14) can be encountered in differently language in Maxine Greene.   Her emphasis on sensing and celebrating the problematic in learning seems to align very much with what Dewey gets at in this chapter.   The "course of development of a situation having a purpose" in a learning environment should be focused on students solving problems that arise from within their experience of that situation.   In this respect the teacher is a learner with the students (previous chapter).   If it's a real situation with students actually identifying and solving problems....then the teacher is learning how a particular "subject matter" an be enacted/embodied....brought to life (even in a classroom).

The teaching of music in so many respects works from traditions about "how to learn".   So getting outside those time-honored paradigms is something that requires great effort for most of us.   What I mean...is thinking differently about how people might learn through their developing judgment about their own experience.   This is essentially a qualitative affair.   But so often it's taught by what seems can inflexible standard of "how it should be"....which tends to replace the students need to judge and to learn how to do that.