What you've described requires technical support from Ariba to customize APC to offer this type of approval process. Your Ariba Customer Engagement Executive, Andrew Hunt, can help you explore options.
SAP Ariba Exchange Community Manager
Thanks for your response. We have engaged Andrew to look into this for us but I was hoping someone else might have implemented similar.
I've had some success with creating an approval flow based on the commodities being added to a shopping cart so I'll carry on exploring whilst I await.