Access referenced entity in Rules using hook_entity_property_info
This works as-is. I actually had a typo in my code that took me a long time to catch. I'm leaving this up if anyone else has the same situation. The final code looked like this (cleaner):

function mymodule_entity_property_info_alter(&amp;$info) {
  $properties = &amp;$info['reply']['properties'];

  $properties['myentity'] = array(
    'label' =&gt; t("My Entity"), 
    'description' =&gt; t("The entity this reply is associated with."), 
    'type' =&gt; 'myentity',
    'required' =&gt; TRUE,
    'schema field' =&gt; 'entity_id',
  );
}