Link Mask Path to Null Object (Child/Parent)

It's hard to figure out exactly what you're trying to do, but this example might help. If you have a mask path and you want to offset all the mask points by an amount equal to the position of a null, you could use an expression like this:

L = thisComp.layer("Null 1");
p = points();
newP = [];
for (i = 0; i < p.length; i++) newP.push(p[i] + L.position);
createPath(newP,inTangents(),outTangents(),isClosed())

If I may, I'd like to add an aside question about masks:

When I apply a Corner Pin to a layer, is there a way to draw masks on this layer in a way that they directly apply to the visible shape of it in master comp?

Example: I have a main background layer A with a TV screen; I have a Layer B (precomp) to embed into the screen, with a Corner Pin applied. But then, if I wanna work on this master comp, like doing a refining mask on a edge of the layer B, the shape that I draw onto what I can see from the master comp doesn't apply on the visible, but it applies just like Layer B were still in fullscreen.

So the option must be to work into the precomp (with master comp locked in Visualizer), but then again I've got some issues with blending options.

If you can help with this also, I'd be the happiest on earth.

Thanks!