Josh -
Easiest thing would be to first add linked fields in PurchaseOrderLine table under Customize > Tables. Since there is already a join from PurchaseOrderLine to PurchaseOrder, you can display linked fields on the PurchaseOrderLine table from the PurchaseOrder table.
Then in the API, you can just pull from the PurchaseOrderLine table and you'll get the results you need.
Either that, or you do two calls - one to get values from PurchaseOrderLine and one to get values from PurchaseOrder.
Paul