I have two ideas on it, although they might not even work.
1. Place scripts in the beginning and end of the route. Once you step on them, they should randomly generate numbers (using the randomnumber command) and if it generates a number that you like, have it erase the flag of the person event who's overworld looks like shaking grass. Of course then you can't walk on it, you'd have to talk to it, but it's roughly the same.
2. The other one I don't even know if it's possible. Start off with the script and the random number thing, and if it's something, there's a change tile command to change the tile so you could change it to a tile with a shaking grass animation. There's also wild encounter data for trees which no-one uses, if you could find a way to link it to your shaking grass tile, you got yourself a working shaking grass.
I think I used Kyledove's black and white tiles for the trees and the grass, if that helps. And it's okay if our hacks look similar, it's the story that'll make ours very different. I think yours will be very distinctly different than mine's, especially since I'm starting to deviate from the normal plot.