You're aware that Rockbox doesn't have its own volume parameters as such, right?
What I mean is, If you use this it will kill media volume across the entire operating system, not just Rockbox. I know that I myself would find this very inconvenient.
Someone may take up this task for you, but I can state with a fair amount of certainty that this almost certainly won't be added to the official builds. Its too obscure, and it breaks another feature that is infinitely more useful, let me explain.
If this were implemented, any touch screen user that makes use of the "mute" touch screen tag is now completely boned. Instead of simply muting playback, playback is now paused, the only way to "fix" this would be to ensure that un-muting also resumed playback, and mute has no business messing around with playback.
I had a brief look, thinking I could handle this for you from userspace with a theme, but it seems that its not possible to set a config value with a conditional statement, it (rather annoyingly) needs to be handled with a touch area, and that makes it absolutely useless for you as if you're going to manipulate a touch area, it may as well just be the pause button.
[Saint]