How quickly are you getting those Prussian Corps concentrated and operational? Had Gneisenau reacted when he first learned of the potential movement of IV Corps to the Belgium frontier, then there was probably a chance for III Corps to do something.
Regardless, if one wanted to play this campaign out in a variable fashion (and I did this for Chickamauga which was largely fought in the woods with very limited line of sight) one could have each side being their final concentration and maneuvers without knowledge of the other through some random system. Sometimes the French arrive to find the allies ready for them, sometimes the French get across the frontier at their chosen line of advance with both allied armies asleep. This campaign has so many possibilities beyond the Brussels-Charleroi bowling alley. Further, there was enough intrigue to allow for the any allied response since, as we have seen, there were plenty of traitors (loyal legitimists) to go around. In real life, they were taking chances with their own lives, but I can see players (who can always just restart) trying all sorts of crazy things - III Corps launches towards Florennes and finds Napoleon cross the Meuse east of Namur!