Well, after restarting kopete, the plugin now works correctly, at least most of the time. The away message is now displayed as configured. But as soon I advance to the next track in my amarok playlist, it once again changes to
Now Listening: %track - %artist (%album)
It doesn't matter, if the former track has finished and the next one is being played or if I manually advance to the next track during playback of the former one. I have to restart kopete again to have the plugin display the message correctly.
So, I am still looking for a solution to get it work properly.