OK, I've made a preliminary fix to give Dukes back the command tab, and will be trying to winnow out the powers Dukes who aren't Lords should have over the next few days.
Tom is away for the week, unfortunately, so the fixes won't go live for at least that long.
It should be possible to just go to
the Secession page directly in the interim if a week's wait will upset your plans. (I don't consider this a URL exploit, because you should have access to it anyway.)