Multi-layout region actions?

I have a multi-layout region with 5 tabs, and would like to launch an action (just an alert box this time!) when a particular tab is selected.

I tried setting an Object - Shown action on an object on the relevant tab layout, but the action never got launched. Maybe because the object doesn't start hidden?
An OnClick action on the same object does launch the script, so I know the script itself is ok. I just need an action that is triggered automatically when the tab is selected.

So is there any way to do this?