Turn on/off expression on specific keyframe

Is it possible to control specific keyframe with expression?
I have an animation that grows from 0 to X and decreases to 0 and I would like to control X using expression.

In that specific instance, probably what I would do would be to create two custom controls with the Edit Controls dialog.

One of the controls would contain the keyframe animation (timing), and the other the magnitude (x). The expression on the control you want to drive would simply be:

timing * x

Thx Bryan, I will try.