The only solution to display a "success" message is via javascript. It could be done by creating a hidden checkbox on the form.
on the OnLoad function in javascript, a function can be made in order to alert the information message whenever the checkbox is not checked, and then check it in the same function (to indicate that the message was displayed, therefore it will not be displayed again) - setvalue and setsubmitmode("always") should be used to update the checkbox from javascript.
This way, whenever the "Create Post Operation" plugin is launched and ran successfully, the form will display a success message (from javascript), and in case of error it will display and exception message (from the plugin).