Errr... It would be tough, but it could be done. It would require a small branch in the step routine, but it is fairly basic ASM in terms of complexity. It can't be done with just a script because scripts lock the player in place. The only way to do it with a script, would be to have a map script which checks the amount of steps since the last time it was used and if so, cause berries to grow. The only problem with this, is the scripts would only be loaded when reentering the map.
Now that I think about it... I have a vague memory of seeing a note in JPAN's hacked engine that said you could have a script run every step. But I'm not sure. In that case, you could designate a var, and add 1 to it each time. When it hits the number of steps you like, it is reset and berries grow.