It's weird. About 10 minutes after I gave up on it and just set my tablet down on my desk, all of a sudden it popped up a SuperSU dialog asking for permission for root for Music2SD. I clicked Grant and then it, basically, seemed to work. At least, it let me choose External Storage.
But then I went into Google Music Player and pinned a song from my own collection. I got a notification that my music download was complete.
I turned off WiFi on my tablet, then went back into GMP and started the song playing. That worked, so I knew it was now local.
But then I went into Astro File Manager to verify where it was stored. I looked in Android/data/com.google.android.music/cache on both the internal storage and the SD card and both were empty!
Then I did a Restart on my tablet, still with WiFi off. When it came back up, I tried to play the same song and it said an Internet connection was required.
So, the real answer is that Music2SD seems totally flaky on my tablet.