How can I link a mask path to a track null?
In After Effects CC, masks can be made into tracking masks. Right click the mask and you should see a context menu with Tracking Mask. Select it and the mask can be tracked to the video it is applied to. The Adobe page about it is here.



This will track the video but the mask will be a rigid shape. If you want the points inside the mask to track individual points in the footage you're going to need a third party extension. This one does the job:

http://aescripts.com/tracker2mask/

Another option is to apply the mask to a solid layer, then make the tracking null the parent of that layer. This can be useful when working with 3D tracked points, because you can mask objects that are rotating in the frame, eg. for logo removal.