Enable Web Users to see all Purchase Requisitions/Orders raised against the budgets they own

We have web users set up as Budget-holders and Buyers depending on their levels of responsibility. The Budget-holders have an assigned set of nominal codes for their department. However in the Purchase Order list available to web users, they can only either see their POs or all POs (and then have a filter applied to show only those raised by them and their buyers). It would be good if there was an automatic filter that could be applied that means they automatically see all POs raised against the nominal codes they 'own' without making all PO details available to them.