Convert a PlaneProjection into a MatrixTransform
<Rectangle Fill="Red" Margin="50">
    <Rectangle.Projection>
        <PlaneProjection RotationX="-40"/>
    </Rectangle.Projection>
</Rectangle>

var matrix = (myElement.Projection as PlaneProjection).ProjectionMatrix;
var matrixProjection = new Matrix3DProjection { ProjectionMatrix = matrix };