Likely would be done as so:
/client/verb/pick_accessaries(var/list/accs as list in list(blah, blah2, blah3))
Client side you can either do a normal combo box with multiple selections enabled, or make something where clicking on an item toggles it's selection normally, so the user doesn't have to hold control. Maybe even add some checkboxs!
The biggest use /tg/ sees this getting is for bitflag vars when map editing, using the var/blah as anything in list() syntax. We currently have to have the mapper provide a string and convert to a bitflag on New(), a way to provide a list to the mapper of possible values that allows for multiple items to be selected in the mapeditor would be much more mapper friendly. However this feature request could even be expanded with direct bitflag support as another input type, ... as bitflag in list(....)