This business of constructing an extensible inventory schema from scratch is apparently not as simple as it might seem. Of course, I didn't really expect it to be simple, but it's about an order of magnitude more abstract than I anticipated. I'm almost ready to start coding, and that's a bit of a daunting project.
Oh, well. If it were too easy, it wouldn't be much fun, right?
I am really surprised there aren't a lot of example schemæ of this sort floating around the net.