Basically, with support of the vehicle stereo mod, pressing a key activates music selected from a GUI, maybe? Example:
I bind my K key to the song "We Will Rock You" it will play through vehicle stereo.
\
Nexus' server command gui can do this.
You can set almost any command to a keybind with it.
However, you will have to select the song, but if you click off "copy", you can just play the song in one keystroke and one mouse click.
I understand this is different from what you want, but this is just an alternate way in case this doesn't get made.