In the XPathconstraint property: "[Show]"
Perhaps a Xpath constraint will help you out?
It sounds as though you should not be using an enumeration at all, but using your lookup table and a relationship between that and your primary object. That way you can use the X-Path constraint as Michel suggests. The association is shown as a drop-down list on your primary object form.
If you really need the value stored as an enumeration in your domain model, you could then set the enum field from the association using a microflow (on change, before commit etc.)
Thanks, but I was using a normal Dropdown, not a Reference Selector. I've changed it now to reference selector, based on the advice and clues you gave me.
It does the job now, but a filter on dropdown selections might be a nice feature in the future?