Unfortunately I didn't have the time to really set my mind to it the last weeks, it was a very busy time. Next month will be hopefully a little bit less hectic, so that I will have the time to get to it...

I noticed that in KDE 4 you have the ability to use more that 4 layouts. To activate this just check the "Spare Layouts" box in "System Settings>Input Devices>Keyboard>Layouts" and add any extra layouts you want. These spare layouts can only be accessed by right-clicking on the layout button in the system tray (or if you assign a shortcut key combination to a layout).
Still no solution in Gnome 3. I really have to sit down and write a program for this (even in KDE -although a big improvement of course- I still don't find it very well organised), but it's not easy to find the time when trying to live in practically bankrupt Greece...