I have a need to limit users to what they can see in built-in dropdown lists on a form UI (for example, customer or transaction, etc). We can do this of course with custom list/record fields by setting filtering, but we do not have access to this filtering mechanism for built-in lists (such as the customer list/record field on a transaction).

Here is the scenario:
I want to limit the user/employee to seeing only customers and transactions of certain partners they are assigned to. We make this assignment in their employee record via a multi-select partner list/record.