Filter Cards for No Users
You can currently filter cards if there are users on them. This feature would filter cards with no users. My userbase doesn't automatically assign users to projects at project creation so this would be helpful for quickly looking for unassigned projects.
I understand the requirement. It will be solved this year when we introduce a full blown Resource Management into KanBo - it will propose people that are not only available but also bring the right skills needed for a task or project (assuming you have entered that information into the KanBo card.
For now I can only recommend to use Search and then use the Users refiner and select "No Users" to find all unassigned cards across multiple boards or in a single board. The only downside is that you have to enter a search phrase first and once you see the results you can delete the search phrase to see all cards with no users assigned.
Does this information help you?