Hi,
That's an interesting use case, and one that can't really be managed using the existing 'add test cases' function.
However... thinking about how you could workaround this problem, if you look at the 'Add Test Cases' screen you'll see that we have a Filter button and we have some choice on what test cases we can see. You might be able to utilise this and add labels to test cases to identify which tests are assigned/not assigned. It's not perfect and it requires some small effort on your part to maintain but there is a Bulk Edit feature to help with that as well, and the Filter is something you can Save also.
