BLT has a busy command, which
The way BLT manages the user interface is by pasting an invisible window over the "busied" frame. The invisible window intercepts all clicks and keystrokes ...
The effect of clock-face cursor can be had in pure Tk by
. configure -cursor watch update # time-consuming action here... . config -cursor {} ;# RS
Example code for the cursor change appears in an old private page [L1 ] as well as the Cookbook [L2 ].