But then the “admin client qube” won’t be able to function as “dom0 managed over ssh”. AFAIK, you can’t e.g. qvm-create ... in a domU, so the “admin client qube” will simply not function as such. Or am I missing other options?
Not completely arbitrary. The service in dom0 that reads the command queue through polling the “admin qube” domU can check if particular commands are allowed and act accordingly.