I'm not sure if we're talking about the same thing, but what I think you're describing is already a big part of LP. For example, create a slider widget, click on it in editing mode, choose the + next to "Value Change," and then you can choose "Adjust Parameter" under the corresponding thing. So "Color Group Actions" for a color's volume, "Audio Source Actions" if you want to adjust an audio source volume, like a synth, etc..