FWIW the solution is:
user@host bin]$ cat mkshaindir #!/bin/dash cd $1 sha512sum * >.sha512sum [user@host bin]$ find /var/ftpd -type d -print0 | xargs -0 -i mkshaindir {}
Note that mkshaindir, for my purposes, is a separate component because there may be a need for me to make a hash of files in a new folder, or of one that was recently changed.
The rest is left as an exercise for the reader.
Note: sha512sum will complain to STDERR about non file input (e.g. directories, block files, etc).