Frank,
Your concerns are not un-warranted either, as you've mentioned yourself, this whole thing involves the installation of unapproved modifications to this equipment.
In saying that though, we use clear plexi-glass covers held in place with insulated studs on a DIN Rail and these have a heap of holes for getting test probes in to fault-find equipment and this also ensures adequate air circulation.
Having things fall into the live busbars or other equipment is not really an issue, because of the way the shields are formed, they have a 45 degree angle at the top and bottom of the part that is held to the studs.
