I'm responding publicly to your private message to me, to keep this discussion open and not have unfinished threads dangling on the mailing list.
You asked if there are other well-defined controls that you can use, as your drop-down only offers those and not the undefined ones. There are plenty of well-defined control messages that FluidSynth itself doesn't act on, the ones you listed are among them, I think. But bear in mind that any well-defined control (just like the balance control) could possibly be enabled in FluidSynth in the future. That's why they are well-defined: they have a very specific purpose, defined by the MIDI specs. And even the undefined or general purpose controls could possibly be used by a particular SoundFont in a modulator, so that there will be side effects in your setup. But from what I have understood from your setup, changing CC8 to any other control doesn't sound like a problem and is probably quickly done.
You wrote that Fluidsynth is "our application" and that we can do what we like. Well, Fluidsynth is most definitely not our application. It's free and open-source software that we contribute to (or in the case of Tom currently act as maintainer for). And we change and extend a piece of software that many other great people have contributed to as well. And Fluidsynth has many users, you included. The reason why we discuss these things on the mailing list is so that people can chime in with their ideas, their knowledge, their reservations, their fears, their own use-cases. To discuss. So no need to play down your part or get defensive. Open discussion is a good thing.