I have not particularly looked into the persona ROM hacking scene but could see it having something with its fanbase (or the greater Shin Megami Tensei fanbase) being what it is. I imagine you would have found that already if such a thing had produced nice game specific tools though
More likely is the same tools as everybody else uses and then going manual and figuring out what encodings, formatting/markup and pointers the game uses. The PS1 does not do anything substantially different to other file system based consoles here to need specialist tools or dismiss others -- it is still table files, markup and pointers just like everything else, or graphics if the game is a real pain (rare for an RPG, normally puzzle games and similarly low text volume affairs). The ROM hacking guide in my signature or any other that covers text hacking will work here.