I have just uploaded a widget to the gallery that displays a coordinate panel along the bottom of the application that shows Map Coordinates, MGRS and Scale. It allows provides check boxes to turn on/off both MGRS and Graticule Grids. Also includes a Zoom to MGRS Function.
Give your map tag an attribute of bottom="25" to ensure that the panel does not obscure any of the map face.
It already shows where the grid is by the form of a rectangle. If you only type in a 6 figure grid the rectangle will be 1km square, if you type in a 10 figure grid it will be 1m square,
