It works with horizontal shutters also. Just have the cord running up and over a pulley to the cock rod on the shutters, and as the reservoir deflates, the cord gets pulled around the pulley and hey presto, an open swell box.
(Of course there is one distinct disadvantage to leaving the swell box open. When some [insert expletive here :angry: ] tries to burn the church building down and the heat blast hits the wonderful 1905 Hill 3M , the heat melts the tops of all the bass pipes in the open swell box. But if the box is left closed, as the choir box was, the pipework remains safe from waves of superheated air.)