Add information to matplotlib Navigation toolbar/status bar?

You simply need to re-assign ax.format_coord, the call back used to draw that label.

See this example from the documentation, as well as In a matplotlib figure window (with imshow), how can I remove, hide, or redefine the displayed position of the mouse? and matplotlib values under cursor

(code lifted directly from example)

"""
Show how to modify the coordinate formatter to report the image "z"
value of the nearest pixel given x and y
"""
import numpy as np
import matplotlib.pyplot as plt
import matplotlib.cm as cm

X = 10*np.random.rand(5,3)

fig = plt.figure()
ax = fig.add_subplot(111)
ax.imshow(X, cmap=cm.jet, interpolation='nearest')

numrows, numcols = X.shape
def format_coord(x, y):
    col = int(x+0.5)
    row = int(y+0.5)
    if col>=0 and col<numcols and row>=0 and row<numrows:
        z = X[row,col]
        return 'x=%1.4f, y=%1.4f, z=%1.4f'%(x, y, z)
    else:
        return 'x=%1.4f, y=%1.4f'%(x, y)

ax.format_coord = format_coord
plt.show()