Nothing. It's out of my control. People will simply have to hex edit the wild data if they want to use the new Pokémon.
As of right now, no. However, the way the ratio is calculated is that any grass tile has a 5% chance (or so) to generate a double wild battle. It does this by picking a number off a table to compare with, so if you created a secondary grass tile, or added another tile to the list, which is only used in a certain area, it would be possible to control the encounter rate.
I'll explain further how this actually works at a later time.