Well, I see now that org-plus-contrib has its issues. Specifically I am hitting the same issue as reported here, which I see has been a problem for nearly 2 years.
Therefore, unless I hear another idea or an objection here, I intend to proceed with getting ox-taskjuggler in isolation onto MELPA, unless its author and/or maintainer would prefer not.
Actually all you need to do is install the org-plus-contrib package from
ELPA, which includes contrib. Then you'll get the latest release of
Org, too. ;)