It's a pretty simple mod to add a 1/4" input jack to the back of the Katana cabinet. You want the kind that cuts off the internal amp connection when you plug a cable into it. That way, you are using the Katana purely as a speaker cab. Obviously, you leave the Katana "off" while using the Marshall head.
This is exactly what I've done with my H&K Edition Blue 60 with a Celestion speaker. I'm using it with my Bogner Alchemist head, and haven't had the H&K actually powered on in years.