Differentiating between invoice templates when adding custom fields

Differentiating between invoice templates when adding custom fields

We are adding HS codes to the items which we would like published on our international customers invoices plus other export data required. We don't need this data on our domestic customer invoices.
We have setup an invoice template for them but it if we make the customized field mandatory in invoices it affects all invoices.
Is it possible to choose an invoice template through a value such as "INVOICE:INTERNATIONAL INVOICE' (name of template)  for example.
Presently the only value acceptable is "INVOICE", "SALES ORDER" etc