Keyframe a replicator 'rate' behaviour

How do you keyframe the 'rate' behaviour used within a replicator? There doesn't appear to be a keyframe option but maybe I'm missing something. I'm wanting the animation to run for a couple of seconds then slow over about a second and then stop. < scratching head >

As an example, if you take the Swoopy Star from the library then open it up, then open the circle up with in it, there is a rate behaviour applied to the circle. Under behaviours there is a rate option. How can that be modified over time?

In this case, as one of several options, I'd disable the Rate parameter behavior and keyframe the Angle parameter manually, as Rate isn't keyframable.