I need to something a bit more complicated though - the drop-down is a custom field on a custom record, of type List/Record (Item) and I need it to look at the line items on a Transaction (specified in ANOTHER custom field on this record) and only allow those items. So I can’t do it with filters, need some sort of saved search I think.

Yeah in my situation I needed a drop down of all the unique inventory items on any sales order for the customer. I added an ad hoc field to the form in a beforeLoad user event, and populated it using a search either there in the user event or during a postSourcing client event (once the customer had been selected).