It's finally done! With the help of our new member Kitaro, it was finally possible to build a good ROM of Bound High!
After all, it was neither a bug in the ISX converter, nor one in the game's source, but a faulty code optimization routine of the VUCC compiler, which could be circumvented with a very small change to the source. Such a little thing, so much hassle!
I'd like to say thank you to everybody who was involved in this and helped to work towards this release, you guys rock!