Effect --> Transition --> Card Wipe. For the rest - no, you can't access mask data with expressions. Only scripts can. It's a moot point, though. If you know the width of a strip or can calculate it in some way, placement will be correct one way or another....
thanks the card effect is pretty much what i should have thought of first. Cheers for the heads up.
You can easily move the anchor points with the Anchor Point tool. It's right there in the toolbar. Just grab the anchor point with that tool and drag. Changing anchor point values in the timeline window causes the layer to move, but dragging with the tool (also called the pan behind tool) doesn't.
Card wipe or even card dance is a good alternative that requires less work but it offers less control in AE's 3D world.