I had a plan to include a JPEG 2000 plugin, based on the original jj2000 code (used in JAI). I even had working code. However, I don't understand the license of the jj2000 code. And my efforts trying to reach out to the original authors or Sun/Oracle who released the JAI code didn't lead to anything. So I left it there.

Nowadays, the format isn't as widely used or important as we first thought. There's currently no plans to include a JPEG 2000 plugin. But let me know if you are interested in helping developing or funding a plugin, and we'll see.