Colby Pender wrote:
1) be proactive about ukemi, don't let myself get locked. Realise when it's at a point it would lock most 'normal' people and submit to the technique or pin.
2) Wear a strap which reduces the range of motion of the joint in order to 'normalise' it.
There will be times when doing #1 would be impossible, like receiving a hard, fast nikkyo. You won't be able to anticipate your joint becoming hyper extended.
On #2, I've never seen a "device" that would offer you sufficient protection while not being restrictively stiff or bulky, or potentially dangerous to your training partners.
My recommendation is good ol' fashioned athletic tape. An orthopedist or sports-medicine specialist should be able to show you some good taping methods.