One Purchase Order line with multiple invoices issue

We use Call-off orders which are PO’s set up with one line in which multiple invoices will be matched too over a period of time. There are two types of Call-off order:

1. An order is created for an amount and NCC does not know how it will be invoiced. An example would be that a Supplier is given a contract for a certain amount and they will perform certain services within that contract.
2. An order is created for multiple services, that if you were to raise it as individual PO lines there could up to around 400 of them.

NCC want to get into a position to use a Bulk Validation process. Call off orders will cause issues as they would not validate and be placed on a system hold. Reason being that invoices need to be in the Not Validated status to run the Bulk Validation process. When an invoice is in the