Journal Notes are stored in the SQL level in the Journal table. There's a ParentLink Category field there you should be able to leverage to pull only those related to a certain type of parent object.
I recognize your ID as one of our ISM customers. I am moving this to the correct subspace. Please note the IT Service Management (v6 and v7) (powered by HEAT) subspace is for the older ITSM product lines from years past (that some folks are still using).