You could modify the /apps/billboard/text GConf setting directly (or any other settings for that matter) - whenever that changes, it forces a Billboard update on-screen.

The Billboard update interval is not configurable, because all updates are based on events, to save battery in the general case (i.e. when events happen less often than the corresponding update interval). There is a non-event-based update interval, and it uses a system-aligned timer (again, to save resources) - so that's not configurable by design.