From a theoretical standpoint, it's not impossible to change even a few tiny things of the Frontier, depending on how GameFreak's implemented it. You can't have different trainer pictures let alone different Brains, but if the trainer name and trainer type are actually typed out in the Battle Recorder, and the Pokémon are also stored in the Recorder data, then you can theoretically have new trainers (from Platinum's existing set) with different sets of Pokémon. This same logic can be applied to the Frontier Brains - that is, assigning them different Pokémon.
And it won't affect co-op battles either, as the trainers in the Frontier are drawn from the host game card. For example, say that Youngster Garret and his team exist in HeartGold only. If you do a Frontier co-op using Platinum, the game will draw the data from HeartGold and volia, the Younger will appear, name and all, in Platinum.
Of course, if the Battle Recorder is disabled outright in the Frontier, then all these restrictions that I've mentioned so far are moot. And if the Battle Recorder can record sprite data, then well, the "must keep old Frontier Brains" logic is dead.