Celing directories and browseable lists of selectable items generated via a search were two new capabilities added in our 2.2.0E release that can be configured on a per-field basis as you note correctly in web-client-config-wcm.xml. Information on this is available on our Forms page on our wiki, and this capability - currently available only as part of an enterprise maintenance release in response to a support case - will be available in our next upcoming 2.9 Community release this June.