JohnS,
I do pushups on my knuckles, that seems to help a little with wrist strength. If you can do pushups on kettlebells, or renegade rows on kettlebells, that will help more. Climbing exercises build tremendous crush strength, which builds overall wrist strength.
As to conditioning the knuckles without damaging the hands, my dad knew this guy who would simply smack a city phone book so many times every day, both with the back of his hand and the front. A friend of mine who does kung-fu used a bag filled with mung beans and did backfists against it. Again, I do a lot of pushups on my knuckles, which I think helps. Part of what you're doing when you condition the hands in this manner is also building pain tolerance, however, as you've already stated, you don't want to go so hard you ruin your hands. Other things that you can think about are setting up an Army surplus duffel bag filled with clothes or other soft material as a heavy bag, and hitting that regularly. For the inside of the hand, lifting weights and doing lots of pullups builds tremendous callouses on striking surfaces inside the hand.
I'm not certain that you can build up the wrist enough so that it doesn't turn if you hit a person wrong, but I suppose nearly anything is possible.
Kent