You should be able to do this with TrackerViz. Make sure it's set to use expressions rather than keyframes, and the mask should update live as you adjust the nulls.

http://aescripts.com/trackerviz/

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To: AE-List@media-motion.tv
Date: Tue, 9 Jul 2013 12:13:26 +1000
From: langmeire@gmail.com
Subject: [AE] Controlling mask positions with nulls

Hi everyone,

I'm working on a project where I really need to be able to control a mask's position with a null or another layer. In other words, if I move a null from point A to point B I'd like the mask to follow. Moving the entire layer that the mask is on is not an option unfortunately. I should also mention that there are multiple masks which need to be controlled within the same comp.

Many thanks