Field Validation for Parent Organization

I'm trying to add field validation to the account object so when the account type= office, I enforce that a parent organization be filled in.

parent organization is a picklist readonly field so it won't let me put any field validation on there.  is there any thing else I can do?

I don't really want to send the modified user an email after the fact as it will get ignored..i'm trying to prevent the save if the data I need isn't filled in.

thanks,

jo-ann