I am very impressed by the "Icon Only" taskbar widget. I use a vertical panel on my netbook because I have a 16:9 display ratio.
But this introduces a new problem: the panel itself can't contain so much.
This is why I am so surprised by the fact that the widget reuses the launcher-button, when it is available, after you startup a program. This saves vertical space.
However, Libreoffice launcher icons are not reused. Both if Libreoffice is installed from the repositories (kubuntu) and if Libreoffice is installed from de site http://www.libreoffice.org.
I tried configuring the "icons only" taskmanager in all sorts of ways but I had no luck.

Does anybody know a solution?

Amazing! I found it out myself. Thanks to this youtube video http://www.youtube.com/watch?v=LtE3XlloLxU please watch it.

under "launcher matching rules" you firstly browse to you application ("launcher"). After that you have to fill in the "window class".

"window name" must remain empty!

In this youtube-video we learn some names of "window-classes".