Ok, so I know this is an old post, but this occured to me only recently, and I would like to leave a comment in here just in case someone else gets as stuck as me again.
Short answer, you can't do that.
"You can use keyframe-looping methods to repeat a series of keyframes. You can use these methods on most properties. Exceptions include properties that can’t be expressed by simple numeric values in the Timeline panel, such as the Source Text property, path shape properties, and the Histogram property for the Levels effect."