Glad to see someone else trying to do something as well! And yeah, I've looked through all the text files and most of the stuff I understood. you can change the profile card and even the background music for settings or rooms through the text files.Hello. It's been a long time since I last used this account, didn't feel the need to actually post.
After a bit of cleanup on my SD card, I started messing around with this. I dumped my retail cart with braindump and loaded a modified romfs with HANS just fine:
(Translation from here, the third line crashes the game as described here)
There's a bunch of parameters stored as plaintext files that are commented (in Japanese). I haven't done it myself, but it should be trivial to do things like modifying the timing windows or the life bar's leniency.
I'm trying to find a way I can open the .dsc files because those are the tracks/notes for the songs on what ever difficulty you play it on.