This could be implemented (by searching a new route to the destination from the current position and then starting navigation from the closest point of the new route).
But the trickier part is:
How to detect that something went wrong and rerouting is needed ?

So that it is not rerouting all the time or failing to reroute when needed.
A few possibilities that come on my mind:

• distance to the next point is increasing for n seconds
• distance from the last point to current position is longer than the distance between the next and previous point
• making some sort of a corridor around the track, leaving the corridor triggers rerouting

Any other ideas how to do this ?
I thing I'll just make clicking the routing info box trigger the rerouting for now.

It is currently enabled together with the visual navigation messages and I the "espeak" package is set as a dependency for modRana.
You can try if espeak works by typing this to xterm:
I strange mechanical voice should great the world :-)

Also, do the routing messages change when you travel the route ?