Not really that advanced compared to what you guys have been up to, but I was asked whether I knew where Steven's double battle team in Emerald was located. A bit of digging later, and I found it. It's not quite in the same format as normal trainers.
Species (2 bytes)
IVs (1 byte)
Level (1 byte)
Nature? (1 byte) - unconfirmed; check this (Metang should be Lonely, Skarmory Impish, and Aggron Adamant)?
EVs? (6 bytes) - unconfirmed; check this?
Padding (1 byte)
Moves (8 bytes)
Unlike ordinary trainers (as far as I can remember), Steven's data appears to include nature and EVs. I bet this could be used to add opposing trainers with stronger Pokémon somehow.
Anyway, a question: this isn't really useful for ROM hacking, but it's related. Could somebody find the formula used to calculate the PIDs of eggs in Emerald if the mother is holding an Everstone? I know the formula without the Everstone already, but I'm stumped as to what happens when one is used.