Stay with me on this...
The Adler Planetarium had this thing where the audience had a button on each armrest, so the audience could "steer" some of the presentation (it was less dumb than it sounded). Before the show, they had a display up that had a little square for each seat (arranged in a grid instead of the circular setup of the room, so it wasn't immediately apparent which was your square). When you pushed the left button, the square turned red, and green for the right.
It took a few minutes, but even with a packed house and a bunch of overcaffenated kids pushing the buttons constantly, you could figure out which one was yours, by just watching the screen and watching for your button pattern.
Since you have even more control over the cursor, I think this will work. If the other cursors are doing apparently purposeful stuff (say, by recording previous paths to clicked buttons), it should be hard for a shoulder surfer to do the same thing, since watching the screen and tracking the mouse is easy for the user (since the mouse is in their hand), but hard for them.