Alright, time for another update, a week later. I've still been fiddling with a/0/0/6 with mixed results. I found the PokeDSPicPlatinumTrainer program and tried using it on that file--it did manage to successfully load the palettes, which is quite nice, but it still renders the images as pure static, as PokeDSPicPlatinum does--the static just happens to be nicely colored.
So as it stands, I'm stuck unless there's a PokeDSPic for D/P that has the same coding that the PlatinumTrainer version does to load palettes. Also, replacing the png files directly--without worrying about the palettes--does work, except the original trainer backsprite doesn't seem to get replaced. Instead, a miscolored Lance just gets glued on top of the first two frames and then disappears for the last two frames (as we might expect, since I still haven't located where the last two frames are).
This is seriously bumming me out. I feel like I'm so close, but... grah.