What I would do is setup authentication. Then put all of these hosts in a hostgroup called clients. By logging into nagios website as a user that is not a contact for the hostgroup "clients", I would therefor see nothing related to clients, and my health status screen would be clean. If I wanted to see "all" then login using one that has all hosts that I am a contact for.
Other than that, no, you can't tell nagios "hey, this host is down, so exclude it from the health status". That would go against everything that nagios is supposed to be for.