Altering Pokémons' HP before fight?

It may be a stupid question or a feature I missed but, is there any way to alter a trainer's pokemon's HP? For instace, having an ace trainer start the battle with a damaged pokémon (say 20/24 HP)?

I figured out I could just make a copy of that pokémon so to have two same pokémons but with different starting HP (if that can even be done) but I was wondering if there's a "less dirty" way to accomplish the same thing (such as a script call to deal X damage to the trainer's pokémons before battle).

Thanks in advance for any help!

Hope I'm not too late with the answer : )
In method pbTrainerBattle (PokemonTrainers script, line 306), there's a line that says

trainer=pbLoadTrainer(trainerid,trainername,trainerparty)

After this line, 'trainer' will contain loaded info about a trainer. Specifically, trainer[2] will contain trainer's Pokemon party. Now, you could play around and add an if block after that line that says something like,

if $alter_trainer_party_hp
for i in 0...trainer[2].length
trainer[2].hp = 0.8 * trainer[2].totalhp
end
end

Which will, if the global variable $alter_trainer_party_hp is set to 'true', set hp of all trainer's pokemon to 80% of total hp. With a few more global and/or game variables, you could customize this to do whatever you like.

Thanks a lot! :D Very helpful, you saved me twice :P