This is something similar to what GNOME provides: in addition to being able to walk through windows of the _current_ application, it would be good to have shortcuts that allow be to switch to another application and exclude any other windows belonging to the current application from the list.
It turns out this option already exists, it just isn't turned on by default: if I check the "only one window per application" option in kcm_tabbox I get the behavior I want.