Yes. - you just need to bind the items and the selected item to properties, then set those properties.
And by properties I mean LrPrefs.prefsForPlugin (for persistent properties) or LrBinding.makePropertyTable (for temporary properties).
in this case, by "expand" i mean open the popup menu in response to something happening programmatically (that is, not me directly clicking on the menu). in the sdk docs i'm not seeing any parameters i can bind to a popup_menu other than items, visible, enable, and value. is there a parameter i can bind that when set equal to true will "open" the popup menu.
hmm, i just discovered simple_list, which may actually do what i need if i can make it small enough