Is it possible to create a live desktop in the Activity Manager which instead of an image for a background, u can use an application such as Midnight Commander? (or the appropriate KDE equivalent, in this case Konqueror or Dolphin) - something similar to the Search + Launch layout available from the Desktop Settings - Plasma Desktop Shell dialogue box (right click context menu>>>default desktop settings).

At present in the Search + Launch desktop there is a button linking to Konqueror; what I'm thinking is that the app (Konqueror or MC) would be running all the time, filling the whole background screen, as if it was an live image, and other apps or windows could run on top if so needed.

Hard to understand I guess, just a idea about how i would like to customize KDE Plasma.

TIA

While you couldn't embed another actual application, you can embed certain widgets. This can be done by changing the layout - known as a "Plasma Containment". Details on writing your own Plasma Containment can be found on KDE Techbase, and on api.kde.org.