Samurize for windows has this feature of creating widgets which are transperent visually as well as to mouse clicks. This is for the first time I had no objection to have a big clock on top of my application windows as this doesnt obstruct clicking any part of application windows. With transperency enabled, this can be a very good added feature for always-on-top plasmoids giving info and not getting in your way of work. Even the notifications impleneted this way (with only small part recieving mouse click) will make them more un-obtrusive to user.
Example:
screenshot didn't capture the mouse pointer. But muose was inside the clocks circle on top of "luckyBackup...." link. The two graphs below are cpu and HDD access waveforms. These are simple ones created by me whcih show the activity but doesn't hinder my work on applications.