Well, basically this is what plasma is doing already. All notifications go
through the central notification system (embedded in the systemtray but
can be had separately as well, yay for modularity). There are no daemons
or apps in the systemtray for battery usage or network management, even
though you can have a plasmoid taking care of those. Even if you use these
plasmoids (be it on a panel or desktop), notifications go through the
notification plasmoid.