Add these functions, filters and manual POIs to your overviewer config file to add a mca file grid

mca_poly_overviewer

def pointDict( coords ):
    coordLabel = ("x", "y", "z")
    return dict(zip(coordLabel, coords))

def mcafilter(poi):
    if poi["id"] == "mca":
        # print poi
        return poi

mcapoi = []

for x in xrange(-20, 21):
    for z in xrange(-20, 21):
        x1 = x * 512
        z1 = z * 512
        bl = (x1, 64, z1)
        tl = (x1 + 511, 64, z1)
        tr = (x1 + 511, 64, z1+ 511)
        br = (x1, 64, z1 + 511)

        square = [bl, tl, tr, br, bl]
        poi = dict(text = "r." + str(x) + "." + str(z) + ".mca",
                    id = "mca",
                    color = "red" if (x % 2 or z % 2) else "red",

                    x = x1+256,
                     y = 64,
                    z = z1 +256,
                    polyline=tuple([pointDict( coords) for coords in square]))
        mcapoi.append(poi)

markers += [ dict(name="mca file", icon="", filterFunction=mcafilter, createInfoWindow=True, checked=True) ]

overmanualpois += mcapoi