While on the Dashboard or in the Query Log, it should be possible to hover over a specific client or domain and view any comments associated with it.
For example, when hovering over the client [192.168.178.1] on the Dashboard, it should display the comment (if available) as specified in the Clients tab.
I understand that the Clients tab only lists hardware addresses, but this shouldn't be an issue, since each IP address is already automatically linked to a hardware address in the Network overview.
Similarly, for domains, it should display comments from the Domains tab if a comment has been set there, and/or show the comment associated with the list the domain originates from.