This is something that I was interested in doing as well so it's interesting to me too, Martin. After looking into it methinks we can't do this not only due to permissions limitations, but also because of complicated logistics. As you're no doubt aware there are only 3 levels of file permissions: user, group, and other. The DH staff are privileged users so they can set up new groups to put us in.
The only way to enable this multi-tiered administration would be to create a group with you as the group owner, within which you could set up less priveleged "subordinate" users who couldn't access each other's files. Seems sort of like what we can do now but it gets complicated if your customers have downstream clients they are in charge of, it would call for another group. That either starts to quickly overload the admins, or it creates a serious security risk by allowing any number of people like us to have that sort of system access.
I believe this is where the administration panel comes into play. It allows users like us to temporarily be delegated higher level admin rights, in a pretty controlled manner. Don't get me wrong I don't claim to be very knowledgeable about networking, but once I thought about this it was easier to see why we can't do it. Just some thoughts.