Is there any way to make the Gross Amount and Tax Amount Item Line Fields on Sales Orders and Cash Sales 'Read Only' (i.e. Disabled) on these Transaction forms (ideally using Workflow)?

I can mark the Amount field Disabled by workflow (to prevent the users entering an invalid total amount for the line), but they can still edit the Gross Amount or Tax Amount fields.

There is no logical reason to allow users to edit these fields (they are calculated from the Rate, Quantity and Tax Rate fields), and being able to disable these would prevent user errors (and potential User 'fraud' trying to bypass pricing restrictions).