The Catweasel MK3 wasn't really designed to work with mice. From the Catweasel MK3 documentation: "Connecting an Amiga or Atari mouse may be possible, but since there are no hardware counters, this would cause heavy CPU load because of the high rate that would be necessary to poll the JoyDat register."
So, unless someone has written a well-behaved mouse driver, your best bet would be to keep a second keyboard plugged into the A1 keyboard port or purchase a PS/2 keyboard terminator.
You could also lobby for an A4000 keyboard to PS/2 convertor. Or, you can upgrade to the Catweasel MK4 when it becomes available, as it should support any combination of Amiga and PS/2 mice and keyboards (assuming software availability).
Trev