Looping back to close this out. Feedback from the development team is that the recommendation given above will not meet the requirement as stated.
The core point is not to create the PR, but rather to ensure that the data from the contract items can be copied to the PR (via drag and drop of contact items).
We have found a solution to achieve this which involves enhancing the method which is triggered when a row or rows in the document overview pane are dragged into the shopping cart in the purchase requisition. Currently the logic in this method checks that the items being dropped originate from a purchase requisition.