Using SearchSQL in Workflow

Hello

Have a client who wants to restrict values in a list for a Adv Search Select field in a case based on the value in another field on the case. Were they doing this via updating the field on the Case Detail screen, then I have a Createscript done that reads the 'Values' of the other case field, and then prepares a SearchSQL statement to get the right filter through to the field.

But, they want to do this in a workflow action where the case field that controls the filter is displayed for an amendment, and then the next field displayed is the Adv Search Select field. I don't seem able to get the same CreateScript to work in this workflow action, so is it not possible to use 'Values' in a Worfklow action Createscript, or is it just that the field isn't set when the whole workflow rule is triggered?

CRM 7.1.g, btw. Thanks in advance for any help.