I have uploaded a tarball of a modified tasks widget here and I'd love for you to try it out for at least a full day.
You will need cmake, a C++ compiler and the kdelibs and kde-workspace headers installed. On OpenSuse, the kde-workspace headers are in the kdebase4-workspace-devel package, for instance, so a simple "zypper install cmake kdebase4-workspace-devel" should hopefully get you most of the way there.
Once you have downloaded the tarball, extract it to somewhere on disk and then create a build directory inside of the resulting directory. From in the build directory do something like: "cmake ../ -DCMAKE_INSTALL_PREFIX=`kde4-config --prefix`".
If all goes smoothly you can do a `make && sudo make install && kbuildsycoca4` type thing and you'll now have a new plasmoid called "Tasks Experiment" in the UI and org.kde.tasksexp behind the scenes. It won't trample on your existing tasks plasmoid or anything.
From there, remove the tasks widget from your panel and in its place put a tasksexp widget. You can even do this from a command line with something like this:
> kquitapp plasma-desktopVoila.
> perl -pi -e "s,plugin=tasks,plugin=org.kde.tasksexp,g" `kde4-config --localprefix`/share/config/plasma-desktop-appletsrc
You can also try it with the "stack icons" grouping by opening up the abstracttaskitem.cpp file and removing the two /s on line 71 so that GROUP_INDICATOR is defined. Then go back into the build directory and do the make / make install dance again, kquitapp plasma-desktop and start it up again.
After a day or two of using it, let me know how it works, or does not work as the case may be, for you.