Cross Business Unit Project Charges when we have No Organization Hierarchy

We have 13 Business Units across the world, which contain 21 Legal Entities. We have these 21 Legal entites setup as Project Expenditure and Project Task Oning Organizations. However, the HCM setp has Departments (Employee Owning Organizations) that have the ability to work anywhere on any Project in any of the 21 Legal Entities. Therefore, there has never been a natural link between these Departments and a unique Legal Entity. Consequently when we come to recieve OTL Transactions (for example) we are unable to cost these as we are holding Cost Rate Schedules at the Business Unit Level, and there