How to mount an APFS volume as a user's home directory?
I found the workaround was to mount the volume on /home and create a subdirectory as my new home directory. As

sudo mount_apfs -o noauto,nobrowse VOLUME_UUID /home
sudo rsync -a /Users/myhome /home


Then change the user's home directory in User &amp; Groups Preference Pane Advanced Options to /home/myhome

I have updated my gist with the complete instructions for mounting an encrypted external drive as a home directory.

UPDATE: After using this for a while I discovered problems with backup software. Notably, Backblaze will not backup custom mount points, and Time Machine will back it up, but the volume is not visible in the restore GUI. In fact, it confusingly displays localsnapshots of the files "underneath" the mount point in /home. 

So I caved and just decided to mount the thing in the default /Volumes. I works swimmingly and the gist has all the details.

Oh and by the way, I used Carbon Copy Cloner to copy the home directory. I recommend using that or equivalent (SuperDuper) as opposed to rsync as the developers of these apps are up-to-date on how to copy all the myriad Apple file attributes, extended attributes, ACLs, flags, etc., etc.