Ajax call failing in Workbench email module
Thanks to Clive's suggestions, I logged the request and post data before the Drupal bootstrap kicked in, and found that it was not Drupal causing the issue. 

The root cause was two different mod_security violations: PCRE limits exceeded and matching rule 300016 (Generic SQL injection protection).  Increasing the PCRE limit and adding an exception for the SQL Injection rule:

&lt;LocationMatch "/system/ajax"&gt;
  SecRuleRemoveById 300016
&lt;/LocationMatch&gt;


Just for the record, I got the idea that Drupal might strip out $_POST data from this comment: https://www.drupal.org/node/2209051#comment-8539311  I came across it trying to troubleshoot this issue earlier. Maybe I misunderstood or took it out of context. This was a hard one for me to figure out, and I was grasping for anything that sounded close to my issue. Thanks for the help, Clive!