I was wondering if you could substitute the music that is playing somehow to include your own...
The music file is the biggest file in the data subfolder with the extension .dec. It's a custom Falcom sound format and I've never messed with it, so I couldn't help ya. I'm sure it's possible to hack the game to use a wave file for the BGM instead since it's using those for sound effects, but I have neither the desire nor the time to mess around for that.
Anyway, looks like it's a hit with the masses! Glad the opportunity came up to getting something working released.