Allow a role to edit a single field on the Item record only

So far, I have used Workflows to apply field level permissions to forms (i.e. allow sales staff to edit customer records, but not allow them to update credit limit etc) - this is relatively easy to do. This also allows us to have a single form for customers which is much easier to manage (initially, before my workflow training we had about 3 different forms for different editing permissions which was a pain - and I could never get the relevant roles to default to the correct form, they always had to select the form manually)

However, the trade store staff want to be able to update the UPC code (bar code) on item records, but the Product Manager does not want them updating anything else.