Thanks, Sandra. As I'm sure you knew, I am aware of the answer - no way, no how are blood suppliers going to "discard" ~10% of their product. But I think it's important to consider the consequences of some of the now routine testing algorithms. No testing mean results are unknown, but once one has information, one may be required to take action. Many transfusion protocols for chronic users involve Rh (C/E) and K matching - there's another batch of donors whose (partial) phenotype is known and considered to be quite immunogenic. It goes on.
I do find it interesting that your system "hides" the K+ status, but openly prints the K- attribute on the label.
Another thought: If the K type of all of the patients were known, they could get the K+ units. The antigen frequencies should match up.