It seems that someone finally found a working workaround for the annoying bery black window bug with proprietary nvidia drivers. Transforming the description there into command line args this results in

  beryl --replace --use-tfp --force-aiglx --xgl-binding settings &
and that’s what I’m currently using. tvtime still runs smoothly (in contrast to the other workaround which was much smaller for me), I currently have a lot of windows open (thunderbird, firefox, more than 40 terminals (xfce-Terminal), tvbrowser, tvtime, scite…) and until now I did not get hit by the black window bug. Performance is a bit slow when moving the terminals around but I guess that’s related to the transparency the terminals use—without that workaround I would have never gotten 40 terminals spawned simultanously :)