in the Settings of the Reference Selector, there is a tab ‘Selectable objects’. You could add a XPath constrint there, as for example
[not(isClosed)] which will provide you a list of the open boxes.
Hope that helps!
Can you provide some more details about the context in which your reference selector is? Because of the *-1 association Asset_Box ist should always only show one box but if it’s not restricted, all boxes should be selectable. To me it sounds like what you need.