I doubt that creating a new document type will help you further.
The purchase order authorization objects are checked if the user wants to display the purchase order, which is possible from within MIGO, e.g. when you display a material document and double click the PO number there.
But those objects are not checked when you post a receipt, as it is quite usual in a bigger company that a receiver has no authority at all to display POs. If those objects would be checked then such receivers could never receive anything until you give them the authority to display purchasing documents.
See also Mauro's (SAP employee) answer in this discussion: Missing Authorization check for MIGOu201D | SCN