I personally believe that if you need a cruise control on a zx-6r, you bought the wrong bike.
With that said, you could disconnect the throttle return spring but that could get you in trouble if something goes wrong while riding. I have seen a few gadgets that allow you to use your palm instead of gripping the throttle but it is not exactly a cruise control. (http://www.wolftrax.com/products/wr/, http://crampbuster.com/)
I looked at the throttlemeister website and from what I understand, it is the same as removing the spring.
In a pinch you can try stuffing a pencil eraser between the throttle sleeve and the bar-end weight, or if you've got access to a decent industrial supply store you might be able to find a larger o-ring that can be rolled down between the bar-end weight and throttle grip. Or you can go with something like the NEP throttle lock.
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