I've seen a few ways to do this. I would try to lock it properly somehow... otherwise you'll end up having to using the same solution that this guy came up with:
It involves a hammer, block and a trained chimp (it's harder to find a trained gorilla). The chimp is mostly trained to sit and the corner and laugh while you try to get the chuck off.
But seriously, I had to resort to gearing my lathe down to the slowest rpm and tapping the chuck jaws *gently* with a plastic block and *small* sledge hammer.
All turned out well, but I sprouted a couple new gray hairs in the process.....