Black 2 / White 2 Repel System in Platinum

[Foxes]

I know the B2W2 repel system where it asks if you want to use another repel once the active one runs out can be implemented fairly easily in FireRed & Emerald by this point, but I was wondering if it could also be implemented in Platinum? I see that at least one ROM hack claims to have implemented it (specifically Pokémon Platinum Plus, though when I downloaded the ROM to test for myself it didn't seem to work), but I was wondering how I'd go about implementing this into my own ROM? Any help is appreciated!