Matt, that idea would work. When I started suggesting things I didn't realise that you didn't have a wall behind the end of the mainline and the staging yard.
I think for train safety, you should put a hole between the rails at each end of the sector plate so a train can't roll off while you are raising or lowering it.
For powering the sector plate, 2 wires soldered into an RCA plug with jacks on the layout to pull power off of, or any sort of 2 wire plug that only plugs one way to protect against polarity shifts causing short circuits would work. I would also put an on/off switch wired to the last 3 feet of both the mainline and the staging yard lead so that you can shut off the power to those track when the sector plate is not hooked up. I'm not sure how high from the floor your main layout or the staging yard will be. If it is low enough, You could pick up a fairly inexpensive tool cart from Harbor Freight, Sears, or a home improvement store to use as the basis of your cart for the sector plate. If a cart is low enoughto fit a collapsed scizzor jack on the cart but below the sector plate and still have it line up with the staging yard, you could bolt the scizzor jack tightly to the bottom of the sector plate and also bolt the base of the jack tightly to the bed of the cart. Then just use the jack to raise and lower your sector plate. Put a couple of guide plates at the end of your benchwork to secure your cart to so that it can't move in relation to your layout when you are transferring trains to or from it, and you should be good to go.