A software solution won't cut it, as the KVM hardware reads the scan code directly from the keyboard before it makes to either machine.
Unfortunately I don't think you're going to find a solution for a 87U. a 103/4 UB would solve it, but the way the 87U's are set up I don't think you're going to be able to use it with your KVM unless you can set your KVM to use another key, or unless your KVM has a hardware switch.