Michael Leupold has been working on a class for kdelibs that can report on keyboard modifier (and mouse button) states, the same thing that the keyboard status widget does internally. The class should appear in KDE 4.3, and this is quite nice since it gives us a platform-neutral way to do things like show when the caps lock is on in dialogs with passwords without reimplementing the code to do that over and over.
So today while the P-man was off at a friend's place I scurried off to a coffee shop and sat down to work on the DataEngine part. Other than half an hour or so spent on some phone calls from friends and answering Plasma bug days questions, I was able to just sit and hack on it while sipping cappuccinos for two hours. The end result is a complete DataEngine plus Services in 198 lines of C++ that is (thanks to plamaengineexplorer) fully tested and working flawlessly.
Now any Plasma component in any language can easily interact with keyboard and mouse button states. You can even latch and lock modifier keys ...
With Michael's code, the whole thing should be less than 650 lines code, which is a 35% savings over the old keyboard kicker applet and done in a way that any KDE app or Plasmoid can take advantage of.