Protective Gloves for bmx / trials
It sounds like you have a kind of injury that is common in skiing. What happens is the ski pole handle gets trapped under the thumb when falling, causing the thumb to bend the wrong way. You can find supports such as this one. I can't speak for that one specifically, but if you look up "skier's thumb" on the internet you should be able to find lots of information on the injury and ways to prevent it.

Also, something like a rollerblade wrist guard might do the job, but I'm not sure how well you could grip the handlebars with something like that.