I don't think you have access to the type of object that is using the postbit in 'postbit_display_complete', so you can not add a test to see if it is a post or a pm.
what you could do is create a 'postbit_factory' plugin, that save the $postbit_type. For example in postbit_factory:
PHP Code:
global $show;
$show['hack_postbit_type'] = $postbit_type;
Then in the 'postbit_display_complete' you would put:
PHP Code:
global $show;
if ($show['hack_postbit_type'] == 'post')
{
$show['reportlink'] = $this->registry->userinfo['userid'];
}
Not tested this, but something like this should work.