Local files revisited: Opening local HTML files (file:///path/to/file) in Chrome on Android 4.x
As I wrote in an addendum to my question, the issue was user error stemming from the less-than-obvious way the Android OS file system works. Everything visible in the file system is under a top-level (the real top) directory called "sdcard".  Thus, file:/// URLs only work for paths like file:///sdcard/foo/bar.

I'll add with questionable relevance that if you haven't installed the app "AirDroid", you should. I found it while looking around for answers to a related problem.