SharePoint 2007: Filtered Lookups

I've come across a requirement which is quite basic, creating a filter on a Lookup field in a List. Basically I have a list of Vacancies, and another list which looks up at the Vacancies list. I want the lookup field to only show those vacancies marked as Open.SharePoint does not support this OOTB, however with a little playing around and using SharePoint Designer you can manage to get this to