I hadn't considered doing this, but I really like the idea so I think I'll give it a try. If you have any other ideas for regional forms, I'd like to hear them! I decided to only go with regional forms for Pokémon that didn't exist in the original Platinum, so that there are no contradictions.
You're right, that was an oversight, but as I checked the script, it turns out there's nothing in SceneConstants that's necessary to change for this. All the changes are handled in SceneElements, so I'm just going to delete the extra file from the download.