Creating a vCenter account and hiding the ESX hosts from the account is the best way to handle this. I'm not sure I understand the challenge in creating this account.
If that route is a strict no-go, the only other option is to not add the vCenter as a data source and instead add each of the ESX hosts you would to monitor and poll the hosts directly. Obviously, this means that Virtualization Manager will not understand the cluster relationships. So, I would not recommend this route.
