I was trying to figure out why we can't eavesdrop on D-Bus reply messages, which contain the notification ID and are required if we want to be able to dismiss notifications and/or detect if they're externally dismissed.

Turns out, it's a QtDbus problem, and you can eavesdrop those if you use the "raw" libdbus. Unfortunately this means a lot of boilerplate code to setup e.g. mainloop integration, etc. :
notificationmonitor.cpp.

I guess I should put that into a library so that all smartwatch programs can benefit. It will be useful to implement the new actionable notifications supported by the new Pebble firmwares.