Due to a limitation in Final Cut, it isn't possible to keyframe individual points in mask shapes.
In this effect, I've added keyframable transform and perspective distort controls to masks.
Here are the controls:
You can also apply an additional mask from another clip - choosing whether to use it to add, subtract or intersect with the mask shape.
If you have two copies of the same clip and apply a mask effect to the top clip, you can use the combination to apply effects to part of a clip's frame: