Oh, I see the balance slider now.
And I get what you mean now, about enabling the DSP.
The SetDspEnabled works exactly like the Controls > DSP Effects menu item, and I can't seem to find a function to enable individual effects in the plugin interface either.
As for reloading the settings file while mb is running, you might have to ask Steven about this. He might read this topic and hopefully give you an answer, but I'm afraid I am not aware of any ways to reload the settings file while mb is running.