Thanks for the tip! The xml generator wasn't specified in the readme and I didn't realize it was there.
I ended up just writing it manually, and after a few headaches (involving files not correctly extracted), I finally got a working disc image on one out of two games I needed to work with (Mega Man X4 worked, Mega Man X5 still doesn't, but it's probably an issue with the files I'm working with...)
Now I have a whole other problem: the STR/XA files are being loaded into the game with some offset (there's like 4-5 seconds of a non related track before the right one starts). I think building a working disc wasn't enough, the game has its own table of contents for XA/STR and now it's all over the place. (Edit: I did), but as for my initial question, mkpsxiso did the trick. I'm gonna put some notes on the first post to whoever runs into this thread looking for the same answers. Thank you so much!
I guess I'll have to figure that one out next