i'd love to see one too.
as to squareness, the edges of the face plate are made to be parallel to the blade at the factory, and they are not adjustable (at least i have never seen one). if they are not parallel, you probably need a new saw.
so couldn't you just clamp a straight board to the piece you are cutting parallel to the cut you want to make? measure the distance from the edge of the blade to the edge of the faceplate, and then measure that distance on both ends of the piece and clamp the board there.
alternatively, bobj has a track and runner setup that makes sure the saw stays on the line.