There is a new update for PSIV. It is now version 2. I managed to uncover quite a few things thanks to Regen. Here's an overview of what's been discovered:
* All maps
* All musics and sounds
* Most of the objects in the field screen; for the battle screen I found out where they are in ROM and where they are stored in memory, but they are organized a bit differently so I will take care of those another time
* Most of the events and all cutscenes.
There is now a constants file as usual with all the IDs of the items, characters and stuff mentioned above. Besides some of the hardware stuff like VDP and DMA are labelled and commented as well as joypad input. There is probably other stuff I forgot to mention...