I have injuries in both wrists that bother me a lot when I play for longer than a few hours at a time. It's part of the reason I stopped playing Defender and StarCraft so much. I have a lifelong background in classical piano, and these injuries were developed primarily in college when I overstrained myself. If any of you have ever seen any videos of me playing Defender, a lot of what is cut out of those clips is me grimacing in pain as I play.
It is very specific in DK what causes my pains: going left and right very quickly, such as when you're doing intense/tight barrel control. This is probably the most unnatural motion for the human wrist to do, and as a result it causes a lot of stress. For this motion, I really don't believe any sort of contraption can stop the physical toll it takes on your wrists. I think the best thing you can do is stretch.
There's a very specific stretch I always do before something wrist-intensive, specifically StarCraft and Defender, where I could be performing literally hundreds of actions per minute. This is a stretch I learned in music school that was taught to me when I first began to show signs of my wrist injury. What you do is extend your arm as far outwards/straight as it will go... think of it as forming a 90 degree angle with your body. You then put your hand straight up, sort of like you were about to push against a solid wall. Then you take your other hand and pull the hand you just stuck straight up backwards, stretching the wrist.
DO NOT DO THIS STRETCH UNLESS YOUR ARM IS COMPLETELY STRAIGHT OUT, YOU WILL DAMAGE YOUR WRIST. But if you do the stretch right, it's very beneficial for keeping these sorts of injuries away and it certainly does help at least me play for much longer periods of time.