Your best bet might be to soak the catalytic converter in a detergent solution and then pressure wash it out.  We offer a mild pH water-based product like this for diesel particulate filters and EGR coolers called Berryman Diesel Parts Soak, Part #0533-05.  To use it for this application, you may be able to fit the entire cat into the bucket.  If not, you could either soak one end for at least 24 hours and then flip over and soak for another 24+ hours or you could cap one end of the cat, pour the product in from the other end, and let that soak for at least 24 hours.  When you go to spray it out, start with low pressure and slowly increase it until the carbon or other debris starts coming loose.  You can perform the soak multiple times, if necessary.  Just make sure you perform a final rinse with clean water.  The cat can then be blown dry, if desired, but it will not hurt anything if installed damp.