Apply Mask to Motion Request

Hi,

I've been loving the ai masking feature! It's great how you can apply the mask tracking to the opacity effect channel. However, I'm wondering if it's possible to add the same feature, allowing you to add mask key frames to the motion effect channel? It would be a huge benefit for reformatting landscape videos to vertical without having to manually keyframe. I know there is an auto-reframe sequence option, but it rarely works when I have an object moving around the frame. So, I feel using the auto mask feature would give a much more accurate motion tracking option if possible. 

Quick note, when I first tested the object mask tool, it actually did track the subject and keep them in frame. Although it only worked the first time I tried it, and hasn't worked since. 

If there's an option or effect I'm overlooking, please do let me know!

Thanks a million!