Answer
The callback procedure automatically receives an app_message_id_ parameter — use that to query the MESSAGE_BODY table directly (WHERE APPLICATION_MESSAGE_ID = app_message_id_), which holds the original request in its MESSAGE_TEXT column and the parameters that were sent in its PARAMETERS column. This avoids the VARCHAR2(1000) size limit of key_ref_ entirely, since you're querying the full message content back out of the database rather than trying to pass it through a size-constrained parameter.