Set Rotate Point for an Action

Brute force (unless someone comes along with a more elegant solution)...

• Select, and duplicate the object to be rotated.
• From the Arrange menu, flip the duplicate copy vertically, then horizontally.
• While still selected, move the copy so that its intended rotation point is on top of the intended rotation point of the original object.
• With the Graphics Inspector, adjust the duplicate's opacity to zero.
• Shift-click on the original object to add it to the selection.
• Choose "group" from the Arrange menu.
• Finally, apply the intended rotation to the grouped object.

Here is a workaround similar to the previous answer.

Try this on a blank slide to see how it works:

1) create two objects and put some space between them
2) make one of the objects completely transparent (invisible)
3) Group the two objects together (one visible, one invisible)
4) Select the Rotate object action

Depending on the position of the invisible object, you can shift the point around which the visible object rotates. Try it, it's straightforward and easy.

Here's a screencast for how to offset the rotation on Keynote objects when creating, for example, an animated clock: http://screencast.com/t/KPYPHHqG