A day and a half later, I have a slightly improved solution that is inspired by this site:
http://www.bidn.com/blogs/BradSchacht/ssis/1575/batch-files-command-lines
In my Execute Process task I have removed the value for the Working Directory. In the batch file (on the remote server), the top line is:
pushd \\myPGP_Server\Path\PGP
(The path is what I was using in the Working Directory field of the Execute Process task.)
This give me the expected encrypted file when I right-click on the package in the package store. When I run as a job the process using the SQL Service Account the task just hangs indefinitely. Sigh.