That's actually really smart, and something that should be looked into.
But, if memory serves correctly, Rock Smash only produced a set type of Pokemon, no matter what route you were in. Like, Geodudes and the like. But, I think that could definitely be put into use.
Another idea could be the tree data in Advance Map.
Since there's no headbutting in FR/LG, etc., you could probably use the wild Pokemon data from and rewrite it to use the behavior byte of the darker grass.