Hello world, here is my problem : I have a map representing air traffic. If a plane raises an alert for some reason, I want that every window which is currently over the map and hiding the alert changes its decoration, in order to warn the user.

The nature of the alert is fully configureable - currently it is a custom X11 atom, which was used in the previous version by a modified fvwm. The program is expected to run on Kwin 4.3. Do you have any ideas on how to proceed ?

xprop -f WM_NAME 8s -set WM_NAME "Warning, jet about to crash" -id <offending_window_id_here>

this will change the title accordingly.

It depends on what tools you can use (ie. to find problematic windows and impact their properties) and how often the problematic window updates the title (overriding this change)
If you want to print the content of a special property, you'll either have to write a decoration (or derive one) to track the property and render it's content.

Alternatively
a) check whether you can update KDE (4.3 is really old)
b) if you can make use of the compositor check a 3rd party plugin named be.clock on how to render "stuff" on top of everything (in this case the time) and control if via dbus

Ok, thanks Luebking. I'll put this in code as soon as possible, and will add some details to the post for the occasion.

When you suggest to upgrade to a more recent Kwin, is it because it's easier to make these dynamic modifications (ie some specific functions have been implemented), or is it just a general advice ? As this program is for professional purpose, any version change must be motivated

General advice.
If KDE 4.3 works for you, that's oc. ok, but please don't report bugs against the version or expect anyony (me) to recall particular behaviors of that version

No problem "Bug spotted, version upgraded", as the saying goes

OK, problem solved by the developement of a specific plugin, which draws above the decoration - not incredibly nice, but functional, and this is all I am looking for !