I've taken a closer look at JPEG2000 support and think I've go something that will work for our first offering. It turns out that JAI support JPEG2000 by including the class files for JJ2000 5.1 in the jai_imageio.jar. When trying out different load options I had JJ2000 5.1 on the class path head of the JAI jars and I could reproduce the runtime exception that Mark was seeing. However if JJ2000 was removed from the classpath the images started showing up.
I only have 12 JPEG2000 sample files but they all load correctly. There is an issue with images that have a CMYK colour space but this is a know issue with other image types as well. The JJ2000 image library can be sped up with the system property -Djj2000.j2k.entropy.encoder.StdEntropyCoder.nthreads=N, where N is 0 by default.
We should look at ImageIO (and possibly JAI ImageIO Tools) again. They've probably come a ways.