It looks like you've found 90% of what you need to do this. While I'm not 100% up to snuff on D-bus either, I do have one pointer for you that may be interesting as an option: Queen BeeCon Widget has D-Bus trigger-able support.

This would allow you to have say an on-desktop widget (or even one per desktop) that would activate when a text starts, and if pressed could send the corresponding stop command? An app would probably be better/faster, but given this as a tool, it could at least be used to quickly test your theory and act as a fill-in until something permanent comes along.