Not the best solution, but for the sake of time, I worked around the problem. I took the URL from the error output:
https://api.github.com/repos/zendframework/zf2/zipball/4a7cf6a4cf791244e14aa0ca49d1f06916b63202
I plugged it into a browser on my Windows-based workstation. On the RHEL6 box, in the home directory of the user that is using composer, there is a .composer directory. I placed the downloaded zip file in this directory:
/home/composeruser/.composer/cache/files/zendframework/zendframework
I then had to rename the zip file to what was called for in the url (adding the zip extension):
/home/composeruser/.composer/cache/files/zendframework/zendframework/4a7cf6a4cf791244e14aa0ca49d1f06916b63202.zip
The next time composer was run, it used the cached file, and moved on. If anyone comes up with a way to avoid pre-caching the files on a RHEL6 box with NSS, let me know.