Hi,
One option would be to send the notes directly from the GOS in ME29N, using the "send" option as shown below.
Here, you can add the note and send to required target - SAP user ID (SAP Inbox) or external email etc.Test the same and revert back.
Another option would be through rejection of release - You may reject the PO release using the "reject" button in the release strategy tab and maintain the rejection notes in any of the PO header texts / create new text to maintain the rejection notes.
If you have workflow, you may send the notification along with rejection notes to the creator through workflow as well. Refer the help doc: Approval Workflows for Purchase Orders and Change Versions - Architecture and Technology - SAP Library and the thread: PO Release / Reject WorkFlow | SCN
Regards,
AKPT