PSP L2 and R2 button mod ain't pretty, gets the job done
It seems a bit of a crime against the PSP's aesthetics, but apparently some folks out there really, really want to do PS1 emulation right. Acid Modder "Electro" managed to finagle L2 and R2 buttons onto his PSP. He rates his hack a 4 out of 5 difficulty level, which basically means we aren't going near this one, but if you know you way around a soldiering iron, his tutorial is fairly detailed. You'll end up with some pretty tiny buttons up top, but it sure beats mapping those L2 and R2 to the analog stick or something even more convoluted.