Ah. ;-( My eventual goal, of course, is to save back to disk with proper metadata (as well as extras, like a real cover, series, etc).
So, it seems like if I really wanted to do this, I'd either need to do a plugin, or just fix the files somewhere else.
I'm still much further along though, so thank you again for your help; with your concrete example, at least now I can make sense of the template documentation.