Contrain Picklist by Grandparent Value

Hi all,

I've got a structure that is essentially like this:

- Case

  |- Correspondence

    |- Correspondence Template

The Correspondence Template being a Picklist on Correspondence.

I want to constrain the selection of Correspondence Template by the Primary Organisation Id of the Grandparent 'Case'.

I've tried:

1. Exposing the Case Primary Org Id through a join on Correspondence
2. Creating a Calculated Field on Correspondence with a calculated expression: "ParentFieldValue ('Organization Id')"

However, when I create a new Correspondence record, the value of the field is NULL until I commit the record - I need the value there in order to contrain the Correspondence Template picklist when picking the template record (there is a Contraint pickmap on SalesFileName).